diff --git a/changelog.d/3987.misc b/changelog.d/3987.misc
new file mode 100644
index 000000000000..d6b501621103
--- /dev/null
+++ b/changelog.d/3987.misc
@@ -0,0 +1 @@
+Disable USE_FROZEN_DICTS for unittests by default.
diff --git a/synapse/events/__init__.py b/synapse/events/__init__.py
index b782af630807..12f1eb0a3ec4 100644
--- a/synapse/events/__init__.py
+++ b/synapse/events/__init__.py
@@ -13,15 +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):
diff --git a/tests/replication/slave/storage/test_events.py b/tests/replication/slave/storage/test_events.py
index db44d33c68cb..41be5d5a1a1d 100644
--- a/tests/replication/slave/storage/test_events.py
+++ b/tests/replication/slave/storage/test_events.py
@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from canonicaljson import encode_canonical_json
+
 from synapse.events import FrozenEvent, _EventInternalMetadata
 from synapse.events.snapshot import EventContext
 from synapse.replication.slave.storage.events import SlavedEventStore
@@ -26,7 +28,9 @@
 
 
 def dict_equals(self, other):
-    return self.__dict__ == other.__dict__
+    me = encode_canonical_json(self._event_dict)
+    them = encode_canonical_json(other._event_dict)
+    return me == them
 
 
 def patch__eq__(cls):
diff --git a/tests/utils.py b/tests/utils.py
index 1ef80e7b79c6..dd347a0c5986 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -136,6 +136,8 @@ def default_config(name):
     config.rc_messages_per_second = 10000
     config.rc_message_burst_count = 10000
 
+    config.use_frozen_dicts = False
+
     # we need a sane default_room_version, otherwise attempts to create rooms will
     # fail.
     config.default_room_version = "1"
@@ -182,7 +184,6 @@ def setup_test_homeserver(
     if config is None:
         config = default_config(name)
 
-    config.use_frozen_dicts = True
     config.ldap_enabled = False
 
     if "clock" not in kargs: