diff --git a/changelog.d/3795.misc b/changelog.d/3795.misc
new file mode 100644
index 000000000000..9f64ee5e2bb2
--- /dev/null
+++ b/changelog.d/3795.misc
@@ -0,0 +1 @@
+Make /sync slightly faster by avoiding needless copies
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index ef20c2296c18..0965cf2fc550 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -1729,17 +1729,17 @@ def _calculate_state(
     event_id_to_key = {
         e: key
         for key, e in itertools.chain(
-            timeline_contains.items(),
-            previous.items(),
-            timeline_start.items(),
-            current.items(),
+            iteritems(timeline_contains),
+            iteritems(previous),
+            iteritems(timeline_start),
+            iteritems(current),
         )
     }
 
-    c_ids = set(e for e in current.values())
-    ts_ids = set(e for e in timeline_start.values())
-    p_ids = set(e for e in previous.values())
-    tc_ids = set(e for e in timeline_contains.values())
+    c_ids = set(e for e in itervalues(current))
+    ts_ids = set(e for e in itervalues(timeline_start))
+    p_ids = set(e for e in itervalues(previous))
+    tc_ids = set(e for e in itervalues(timeline_contains))
 
     # If we are lazyloading room members, we explicitly add the membership events
     # for the senders in the timeline into the state block returned by /sync,