diff --git a/synapse/util/expiringcache.py b/synapse/util/expiringcache.py
index fb5b27ad1..3c8409b16 100644
--- a/synapse/util/expiringcache.py
+++ b/synapse/util/expiringcache.py
@@ -55,7 +55,7 @@ class ExpiringCache(object):
         def f():
             self._prune_cache()
 
-        self._clock.looping_call(f, self._expiry_ms)
+        self._clock.looping_call(f, self._expiry_ms/2)
 
     def __setitem__(self, key, value):
         now = self._clock.time_msec()