Use the presence cachemap ordering to early-abort the iteration loop

synapse/handlers/presence.py

@@ -875,10 +875,15 @@ class PresenceEventSource(object):
 
         updates = []
         # TODO(paul): use a DeferredList ? How to limit concurrency.
-        for observed_user in cachemap.keys():
+        for observed_user in reversed(cachemap.keys()):
             cached = cachemap[observed_user]
 
-            if cached.serial <= from_key or cached.serial > max_serial:
+            # Since this is ordered in descending order of serial, we can just
+            # stop once we've seen enough
+            if cached.serial <= from_key:
+                break
+
+            if cached.serial > max_serial:
                 continue
 
             if not (yield self.is_visible(observer_user, observed_user)):