diff --git a/benchmark.py b/benchmark.py
new file mode 100644
index 00000000..247baeb9
--- /dev/null
+++ b/benchmark.py
@@ -0,0 +1,182 @@
+#!/usr/bin/env python
+
+import timeit
+
+
+def cprofile_main():
+    from pymongo import Connection
+    connection = Connection()
+    connection.drop_database('timeit_test')
+    connection.disconnect()
+
+    from mongoengine import Document, DictField, connect
+    connect("timeit_test")
+
+    class Noddy(Document):
+        fields = DictField()
+
+    for i in xrange(1):
+        noddy = Noddy()
+        for j in range(20):
+            noddy.fields["key" + str(j)] = "value " + str(j)
+        noddy.save()
+
+
+def main():
+    """
+    0.4 Performance Figures ...
+
+    ----------------------------------------------------------------------------------------------------
+    Creating 10000 dictionaries - Pymongo
+    1.1141769886
+    ----------------------------------------------------------------------------------------------------
+    Creating 10000 dictionaries - MongoEngine
+    2.37724113464
+    ----------------------------------------------------------------------------------------------------
+    Creating 10000 dictionaries - MongoEngine, safe=False, validate=False
+    1.92479610443
+
+    0.5.X
+    ----------------------------------------------------------------------------------------------------
+    Creating 10000 dictionaries - Pymongo
+    1.10552310944
+    ----------------------------------------------------------------------------------------------------
+    Creating 10000 dictionaries - MongoEngine
+    16.5169169903
+    ----------------------------------------------------------------------------------------------------
+    Creating 10000 dictionaries - MongoEngine, safe=False, validate=False
+    14.9446101189
+    ----------------------------------------------------------------------------------------------------
+    Creating 10000 dictionaries - MongoEngine, safe=False, validate=False, cascade=False
+    14.912801981
+    ----------------------------------------------------------------------------------------------------
+    Creating 10000 dictionaries - MongoEngine, force=True
+    14.9617750645
+
+    Performance
+    ----------------------------------------------------------------------------------------------------
+    Creating 10000 dictionaries - Pymongo
+    1.10072994232
+    ----------------------------------------------------------------------------------------------------
+    Creating 10000 dictionaries - MongoEngine
+    5.27341103554
+    ----------------------------------------------------------------------------------------------------
+    Creating 10000 dictionaries - MongoEngine, safe=False, validate=False
+    4.49365401268
+    ----------------------------------------------------------------------------------------------------
+    Creating 10000 dictionaries - MongoEngine, safe=False, validate=False, cascade=False
+    4.43459296227
+    ----------------------------------------------------------------------------------------------------
+    Creating 10000 dictionaries - MongoEngine, force=True
+    4.40114378929
+    """
+
+    setup = """
+from pymongo import Connection
+connection = Connection()
+connection.drop_database('timeit_test')
+"""
+
+    stmt = """
+from pymongo import Connection
+connection = Connection()
+
+db = connection.timeit_test
+noddy = db.noddy
+
+for i in xrange(10000):
+    example = {'fields': {}}
+    for j in range(20):
+        example['fields']["key"+str(j)] = "value "+str(j)
+
+    noddy.insert(example)
+
+myNoddys = noddy.find()
+[n for n in myNoddys] # iterate
+"""
+
+    print "-" * 100
+    print """Creating 10000 dictionaries - Pymongo"""
+    t = timeit.Timer(stmt=stmt, setup=setup)
+    print t.timeit(1)
+
+    setup = """
+from pymongo import Connection
+connection = Connection()
+connection.drop_database('timeit_test')
+connection.disconnect()
+
+from mongoengine import Document, DictField, connect
+connect("timeit_test")
+
+class Noddy(Document):
+    fields = DictField()
+"""
+
+    stmt = """
+for i in xrange(10000):
+    noddy = Noddy()
+    for j in range(20):
+        noddy.fields["key"+str(j)] = "value "+str(j)
+    noddy.save()
+
+myNoddys = Noddy.objects()
+[n for n in myNoddys] # iterate
+"""
+
+    print "-" * 100
+    print """Creating 10000 dictionaries - MongoEngine"""
+    t = timeit.Timer(stmt=stmt, setup=setup)
+    print t.timeit(1)
+
+    stmt = """
+for i in xrange(10000):
+    noddy = Noddy()
+    for j in range(20):
+        noddy.fields["key"+str(j)] = "value "+str(j)
+    noddy.save(safe=False, validate=False)
+
+myNoddys = Noddy.objects()
+[n for n in myNoddys] # iterate
+"""
+
+    print "-" * 100
+    print """Creating 10000 dictionaries - MongoEngine, safe=False, validate=False"""
+    t = timeit.Timer(stmt=stmt, setup=setup)
+    print t.timeit(1)
+
+
+    stmt = """
+for i in xrange(10000):
+    noddy = Noddy()
+    for j in range(20):
+        noddy.fields["key"+str(j)] = "value "+str(j)
+    noddy.save(safe=False, validate=False, cascade=False)
+
+myNoddys = Noddy.objects()
+[n for n in myNoddys] # iterate
+"""
+
+    print "-" * 100
+    print """Creating 10000 dictionaries - MongoEngine, safe=False, validate=False, cascade=False"""
+    t = timeit.Timer(stmt=stmt, setup=setup)
+    print t.timeit(1)
+
+    stmt = """
+for i in xrange(10000):
+    noddy = Noddy()
+    for j in range(20):
+        noddy.fields["key"+str(j)] = "value "+str(j)
+    noddy.save(force_insert=True, safe=False, validate=False, cascade=False)
+
+myNoddys = Noddy.objects()
+[n for n in myNoddys] # iterate
+"""
+
+    print "-" * 100
+    print """Creating 10000 dictionaries - MongoEngine, force=True"""
+    t = timeit.Timer(stmt=stmt, setup=setup)
+    print t.timeit(1)
+
+if __name__ == "__main__":
+    main()
diff --git a/mongoengine/base.py b/mongoengine/base.py
index b4214017..a95339eb 100644
--- a/mongoengine/base.py
+++ b/mongoengine/base.py
@@ -153,15 +153,6 @@ class BaseField(object):
             if callable(value):
                 value = value()
 
-        # Convert lists / values so we can watch for any changes on them
-        if isinstance(value, (list, tuple)) and not isinstance(value, BaseList):
-            observer = DataObserver(instance, self.name)
-            value = BaseList(value, observer)
-            instance._data[self.name] = value
-        elif isinstance(value, dict) and not isinstance(value, BaseDict):
-            observer = DataObserver(instance, self.name)
-            value = BaseDict(value, observer)
-            instance._data[self.name] = value
         return value
 
     def __set__(self, instance, value):
@@ -231,6 +222,7 @@ class ComplexBaseField(BaseField):
     """
 
     field = None
+    _dereference = False
 
     def __get__(self, instance, owner):
         """Descriptor to automatically dereference references.
@@ -239,11 +231,39 @@ class ComplexBaseField(BaseField):
             # Document class being used rather than a document object
             return self
 
-        from dereference import dereference
-        instance._data[self.name] = dereference(
-            instance._data.get(self.name), max_depth=1, instance=instance, name=self.name
-        )
-        return super(ComplexBaseField, self).__get__(instance, owner)
+        if not self._dereference and instance._initialised:
+            from dereference import dereference
+            self._dereference = dereference  # Cached
+            instance._data[self.name] = self._dereference(
+                instance._data.get(self.name), max_depth=1, instance=instance,
+                name=self.name
+            )
+
+        value = super(ComplexBaseField, self).__get__(instance, owner)
+
+        # Convert lists / values so we can watch for any changes on them
+        if isinstance(value, (list, tuple)) and not isinstance(value, BaseList):
+            value = BaseList(value, instance, self.name)
+            instance._data[self.name] = value
+        elif isinstance(value, dict) and not isinstance(value, BaseDict):
+            value = BaseDict(value, instance, self.name)
+            instance._data[self.name] = value
+
+        if self._dereference and instance._initialised and \
+            isinstance(value, (BaseList, BaseDict)) and not value._dereferenced:
+            value = self._dereference(
+                value, max_depth=1, instance=instance, name=self.name
+            )
+            value._dereferenced = True
+            instance._data[self.name] = value
+
+        return value
+
+    def __set__(self, instance, value):
+        """Descriptor for assigning a value to a field in a document.
+        """
+        instance._data[self.name] = value
+        instance._mark_as_changed(self.name)
 
     def to_python(self, value):
         """Convert a MongoDB-compatible type to a Python type.
@@ -727,12 +747,13 @@ class BaseDocument(object):
 
     _dynamic = False
     _created = True
+    _dynamic_lock = True
+    _initialised = False
 
     def __init__(self, **values):
         signals.pre_init.send(self.__class__, document=self, values=values)
 
         self._data = {}
-        self._initialised = False
 
         # Assign default values to instance
         for attr_name, field in self._fields.items():
@@ -754,18 +775,19 @@ class BaseDocument(object):
 
         # Set any get_fieldname_display methods
         self.__set_field_display()
-        # Flag initialised
-        self._initialised = True
 
         if self._dynamic:
+            self._dynamic_lock = False
             for key, value in dynamic_data.items():
                 setattr(self, key, value)
 
+        # Flag initialised
+        self._initialised = True
         signals.post_init.send(self.__class__, document=self)
 
     def __setattr__(self, name, value):
         # Handle dynamic data only if an initialised dynamic document
-        if self._dynamic and getattr(self, '_initialised', False):
+        if self._dynamic and not self._dynamic_lock:
 
             field = None
             if not hasattr(self, name) and not name.startswith('_'):
@@ -825,11 +847,9 @@ class BaseDocument(object):
 
         # Convert lists / values so we can watch for any changes on them
         if isinstance(value, (list, tuple)) and not isinstance(value, BaseList):
-            observer = DataObserver(self, name)
-            value = BaseList(value, observer)
+            value = BaseList(value, self, name)
         elif isinstance(value, dict) and not isinstance(value, BaseDict):
-            observer = DataObserver(self, name)
-            value = BaseDict(value, observer)
+            value = BaseDict(value, self, name)
 
         return value
 
@@ -1147,33 +1167,25 @@ class BaseDocument(object):
             return hash(self.pk)
 
 
-class DataObserver(object):
-
-    __slots__ = ["instance", "name"]
-
-    def __init__(self, instance, name):
-        self.instance = instance
-        self.name = name
-
-    def updated(self):
-        if hasattr(self.instance, '_mark_as_changed'):
-            self.instance._mark_as_changed(self.name)
-
-
 class BaseList(list):
     """A special list so we can watch any changes
     """
 
-    def __init__(self, list_items, observer):
-        self.observer = observer
+    _dereferenced = False
+    _instance = None
+    _name = None
+
+    def __init__(self, list_items, instance, name):
+        self._instance = instance
+        self._name = name
         super(BaseList, self).__init__(list_items)
 
     def __setitem__(self, *args, **kwargs):
-        self._updated()
+        self._mark_as_changed()
         super(BaseList, self).__setitem__(*args, **kwargs)
 
     def __delitem__(self, *args, **kwargs):
-        self._updated()
+        self._mark_as_changed()
         super(BaseList, self).__delitem__(*args, **kwargs)
 
     def __getstate__(self):
@@ -1182,90 +1194,94 @@ class BaseList(list):
 
     def __setstate__(self, state):
         self = state
+        return self
 
     def append(self, *args, **kwargs):
-        self._updated()
+        self._mark_as_changed()
         return super(BaseList, self).append(*args, **kwargs)
 
     def extend(self, *args, **kwargs):
-        self._updated()
+        self._mark_as_changed()
         return super(BaseList, self).extend(*args, **kwargs)
 
     def insert(self, *args, **kwargs):
-        self._updated()
+        self._mark_as_changed()
         return super(BaseList, self).insert(*args, **kwargs)
 
     def pop(self, *args, **kwargs):
-        self._updated()
+        self._mark_as_changed()
         return super(BaseList, self).pop(*args, **kwargs)
 
     def remove(self, *args, **kwargs):
-        self._updated()
+        self._mark_as_changed()
         return super(BaseList, self).remove(*args, **kwargs)
 
     def reverse(self, *args, **kwargs):
-        self._updated()
+        self._mark_as_changed()
         return super(BaseList, self).reverse(*args, **kwargs)
 
     def sort(self, *args, **kwargs):
-        self._updated()
+        self._mark_as_changed()
         return super(BaseList, self).sort(*args, **kwargs)
 
-    def _updated(self):
-        try:
-            self.observer.updated()
-        except AttributeError:
-            pass
+    def _mark_as_changed(self):
+        if hasattr(self._instance, '_mark_as_changed'):
+            self._instance._mark_as_changed(self._name)
 
 
 class BaseDict(dict):
     """A special dict so we can watch any changes
     """
 
-    def __init__(self, dict_items, observer):
-        self.observer = observer
+    _dereferenced = False
+    _instance = None
+    _name = None
+
+    def __init__(self, dict_items, instance, name):
+        self._instance = instance
+        self._name = name
         super(BaseDict, self).__init__(dict_items)
 
     def __setitem__(self, *args, **kwargs):
-        self._updated()
+        self._mark_as_changed()
         super(BaseDict, self).__setitem__(*args, **kwargs)
 
     def __delete__(self, *args, **kwargs):
-        self._updated()
+        self._mark_as_changed()
         super(BaseDict, self).__delete__(*args, **kwargs)
 
     def __delitem__(self, *args, **kwargs):
-        self._updated()
+        self._mark_as_changed()
         super(BaseDict, self).__delitem__(*args, **kwargs)
 
     def __delattr__(self, *args, **kwargs):
-        self._updated()
+        self._mark_as_changed()
         super(BaseDict, self).__delattr__(*args, **kwargs)
 
     def __getstate__(self):
-        self.observer = None
+        self.instance = None
+        self._dereferenced = False
         return self
 
     def __setstate__(self, state):
         self = state
+        return self
 
     def clear(self, *args, **kwargs):
-        self._updated()
+        self._mark_as_changed()
         super(BaseDict, self).clear(*args, **kwargs)
 
     def pop(self, *args, **kwargs):
-        self._updated()
+        self._mark_as_changed()
         super(BaseDict, self).pop(*args, **kwargs)
 
     def popitem(self, *args, **kwargs):
-        self._updated()
+        self._mark_as_changed()
         super(BaseDict, self).popitem(*args, **kwargs)
 
-    def _updated(self):
-        try:
-            self.observer.updated()
-        except AttributeError:
-            pass
+    def _mark_as_changed(self):
+        if hasattr(self._instance, '_mark_as_changed'):
+            self._instance._mark_as_changed(self._name)
 
 if sys.version_info < (2, 5):
     # Prior to Python 2.5, Exception was an old-style class
diff --git a/mongoengine/dereference.py b/mongoengine/dereference.py
index 3dac92b5..40ceb49c 100644
--- a/mongoengine/dereference.py
+++ b/mongoengine/dereference.py
@@ -1,7 +1,6 @@
 import pymongo
 
-from base import (BaseDict, BaseList, DataObserver,
-                  TopLevelDocumentMetaclass, get_document)
+from base import (BaseDict, BaseList, TopLevelDocumentMetaclass, get_document)
 from fields import ReferenceField
 from connection import get_db
 from queryset import QuerySet
@@ -134,11 +133,10 @@ class DeReference(object):
                 return items
 
             if instance:
-                observer = DataObserver(instance, name)
                 if isinstance(items, dict):
-                    return BaseDict(items, observer)
+                    return BaseDict(items, instance, name)
                 else:
-                    return BaseList(items, observer)
+                    return BaseList(items, instance, name)
 
         if isinstance(items, (dict, pymongo.son.SON)):
             if '_ref' in items:
@@ -183,10 +181,9 @@ class DeReference(object):
                 data[k] = self.object_map.get(v.id, v)
 
         if instance and name:
-            observer = DataObserver(instance, name)
             if is_list:
-                return BaseList(data, observer)
-            return BaseDict(data, observer)
+                return BaseList(data, instance, name)
+            return BaseDict(data, instance, name)
         depth += 1
         return data
 
diff --git a/mongoengine/document.py b/mongoengine/document.py
index 59961434..686945c2 100644
--- a/mongoengine/document.py
+++ b/mongoengine/document.py
@@ -1,6 +1,6 @@
 from mongoengine import signals
 from base import (DocumentMetaclass, TopLevelDocumentMetaclass, BaseDocument,
-                  BaseDict, BaseList, DataObserver)
+                  BaseDict, BaseList)
 from queryset import OperationError
 from connection import get_db, DEFAULT_CONNECTION_NAME
 
@@ -304,12 +304,10 @@ class Document(BaseDocument):
         """
         if isinstance(value, BaseDict):
             value = [(k, self._reload(k, v)) for k, v in value.items()]
-            observer = DataObserver(self, key)
-            value = BaseDict(value, observer)
+            value = BaseDict(value, self, key)
         elif isinstance(value, BaseList):
             value = [self._reload(key, v) for v in value]
-            observer = DataObserver(self, key)
-            value = BaseList(value, observer)
+            value = BaseList(value, self, key)
         elif isinstance(value, (EmbeddedDocument, DynamicEmbeddedDocument)):
             value._changed_fields = []
         return value
diff --git a/mongoengine/fields.py b/mongoengine/fields.py
index 2f5911f9..2e4411a2 100644
--- a/mongoengine/fields.py
+++ b/mongoengine/fields.py
@@ -11,7 +11,7 @@ import uuid
 
 from base import (BaseField, ComplexBaseField, ObjectIdField,
                   ValidationError, get_document)
-from queryset import DO_NOTHING
+from queryset import DO_NOTHING, QuerySet
 from document import Document, EmbeddedDocument
 from connection import get_db, DEFAULT_CONNECTION_NAME
 from operator import itemgetter
@@ -491,7 +491,7 @@ class ListField(ComplexBaseField):
     def validate(self, value):
         """Make sure that a list of valid fields is being used.
         """
-        if (not isinstance(value, (list, tuple)) or
+        if (not isinstance(value, (list, tuple, QuerySet)) or
             isinstance(value, basestring)):
             self.error('Only lists and tuples may be used in a list field')
         super(ListField, self).validate(value)