diff --git a/synapse/metrics/metric.py b/synapse/metrics/metric.py

index 341043952a..1721c3705b 100644
--- a/synapse/metrics/metric.py
+++ b/synapse/metrics/metric.py
@@ -15,6 +15,7 @@
 
 
 from itertools import chain
+from collections import Counter
 
 
 # TODO(paul): I can't believe Python doesn't have one of these
@@ -52,30 +53,29 @@ class CounterMetric(BaseMetric):
     """The simplest kind of metric; one that stores a monotonically-increasing
     integer that counts events."""
 
+    __slots__ = ("counts")
+
     def __init__(self, *args, **kwargs):
         super(CounterMetric, self).__init__(*args, **kwargs)
 
-        self.counts = {}
+        self.counts = Counter()
 
         # Scalar metrics are never empty
         if self.is_scalar():
             self.counts[()] = 0
 
     def inc_by(self, incr, *values):
-        if len(values) != self.dimension():
-            raise ValueError(
-                "Expected as many values to inc() as labels (%d)" % (self.dimension())
-            )
+        # if len(values) != self.dimension():
+        #     raise ValueError(
+        #         "Expected as many values to inc() as labels (%d)" % (self.dimension())
+        #     )
 
         # TODO: should assert that the tag values are all strings
 
-        if values not in self.counts:
-            self.counts[values] = incr
-        else:
-            self.counts[values] += incr
+        self.counts[values] += incr
 
     def inc(self, *values):
-        self.inc_by(1, *values)
+        self.counts[values] += 1
 
     def render_item(self, k):
         return ["%s%s %d" % (self.name, self._render_key(k), self.counts[k])]