diff --git a/synapse/state/__init__.py b/synapse/state/__init__.py
index b914a5ba03..4b136b3054 100644
--- a/synapse/state/__init__.py
+++ b/synapse/state/__init__.py
@@ -40,7 +40,8 @@ logger = logging.getLogger(__name__)
 
 # Metrics for number of state groups involved in a resolution.
 state_groups_histogram = Histogram(
-    "synapse_state_number_state_groups", "",
+    "synapse_state_number_state_groups_in_resolution",
+    "Number of state groups used when performing a state resolution",
     buckets=(1, 2, 3, 5, 7, 10, 15, 20, 50, 100, 200, 500, "+Inf"),
 )
 
diff --git a/synapse/storage/events.py b/synapse/storage/events.py
index f2374bc3f4..9903786c48 100644
--- a/synapse/storage/events.py
+++ b/synapse/storage/events.py
@@ -76,14 +76,16 @@ state_delta_reuse_delta_counter = Counter(
 
 # The number of forward extremities for each new event.
 forward_extremities_counter = Histogram(
-    "synapse_storage_events_forward_extremities_persisted", "",
+    "synapse_storage_events_forward_extremities_persisted",
+    "Number of forward extremities for each new event",
     buckets=(1, 2, 3, 5, 7, 10, 15, 20, 50, 100, 200, 500, "+Inf")
 )
 
 # The number of stale forward extremities for each new event. Stale extremities
 # are those that were in the previous set of extremities as well as the new.
 stale_forward_extremities_counter = Histogram(
-    "synapse_storage_events_stale_forward_extremities_persisted", "",
+    "synapse_storage_events_stale_forward_extremities_persisted",
+    "Number of unchanged forward extremities for each new event",
     buckets=(0, 1, 2, 3, 5, 7, 10, 15, 20, 50, 100, 200, 500, "+Inf")
 )
 
@@ -583,7 +585,7 @@ class EventsStore(
         # (e.g. we ignore backfill/outliers/etc)
         if result != latest_event_ids:
             forward_extremities_counter.observe(len(result))
-            stale = set(latest_event_ids) & result
+            stale = latest_event_ids & result
             stale_forward_extremities_counter.observe(len(stale))
 
         defer.returnValue(result)