diff --git a/synapse/events/__init__.py b/synapse/events/__init__.py

index 51f9084b90..12f1eb0a3e 100644
--- a/synapse/events/__init__.py
+++ b/synapse/events/__init__.py
@@ -13,13 +13,22 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import os
+from distutils.util import strtobool
+
+import six
+
 from synapse.util.caches import intern_dict
 from synapse.util.frozenutils import freeze
 
 # Whether we should use frozen_dict in FrozenEvent. Using frozen_dicts prevents
-# bugs where we accidentally share e.g. signature dicts. However, converting
-# a dict to frozen_dicts is expensive.
-USE_FROZEN_DICTS = True
+# bugs where we accidentally share e.g. signature dicts. However, converting a
+# dict to frozen_dicts is expensive.
+#
+# NOTE: This is overridden by the configuration by the Synapse worker apps, but
+# for the sake of tests, it is set here while it cannot be configured on the
+# homeserver object itself.
+USE_FROZEN_DICTS = strtobool(os.environ.get("SYNAPSE_USE_FROZEN_DICTS", "0"))
 
 
 class _EventInternalMetadata(object):
@@ -147,6 +156,9 @@ class EventBase(object):
     def items(self):
         return list(self._event_dict.items())
 
+    def keys(self):
+        return six.iterkeys(self._event_dict)
+
 
 class FrozenEvent(EventBase):
     def __init__(self, event_dict, internal_metadata_dict={}, rejected_reason=None):