diff --git a/synapse/handlers/presence.py b/synapse/handlers/presence.py
index 1edab0549..28688d532 100644
--- a/synapse/handlers/presence.py
+++ b/synapse/handlers/presence.py
@@ -26,6 +26,7 @@ import synapse.metrics
 from ._base import BaseHandler
 
 import logging
+from collections import OrderedDict
 
 
 logger = logging.getLogger(__name__)
@@ -143,7 +144,7 @@ class PresenceHandler(BaseHandler):
         self._remote_offline_serials = []
 
         # map any user to a UserPresenceCache
-        self._user_cachemap = {}
+        self._user_cachemap = OrderedDict()  # keep them sorted by serial
         self._user_cachemap_latest_serial = 0
 
         metrics.register_callback(
@@ -165,6 +166,14 @@ class PresenceHandler(BaseHandler):
         else:
             return UserPresenceCache()
 
+    def _bump_serial(self, user=None):
+        self._user_cachemap_latest_serial += 1
+
+        if user:
+            # Move to end
+            cache = self._user_cachemap.pop(user)
+            self._user_cachemap[user] = cache
+
     def registered_user(self, user):
         return self.store.create_presence(user.localpart)
 
@@ -300,7 +309,7 @@ class PresenceHandler(BaseHandler):
     def changed_presencelike_data(self, user, state):
         statuscache = self._get_or_make_usercache(user)
 
-        self._user_cachemap_latest_serial += 1
+        self._bump_serial(user=user)
         statuscache.update(state, serial=self._user_cachemap_latest_serial)
 
         return self.push_presence(user, statuscache=statuscache)
@@ -322,7 +331,7 @@ class PresenceHandler(BaseHandler):
 
             # No actual update but we need to bump the serial anyway for the
             # event source
-            self._user_cachemap_latest_serial += 1
+            self._bump_serial()
             statuscache.update({}, serial=self._user_cachemap_latest_serial)
 
             self.push_update_to_local_and_remote(
@@ -704,7 +713,7 @@ class PresenceHandler(BaseHandler):
 
             statuscache = self._get_or_make_usercache(user)
 
-            self._user_cachemap_latest_serial += 1
+            self._bump_serial(user=user)
             statuscache.update(state, serial=self._user_cachemap_latest_serial)
 
             if not observers and not room_ids:
@@ -864,10 +873,15 @@ 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)):