diff -r b7c0137cccbe Lib/collections/__init__.py
--- a/Lib/collections/__init__.py	Thu Mar 26 23:50:57 2015 +0100
+++ b/Lib/collections/__init__.py	Fri Mar 27 10:24:52 2015 +0200
@@ -668,14 +668,8 @@ class Counter(dict):
         '''
         if not isinstance(other, Counter):
             return NotImplemented
-        result = Counter()
-        for elem, count in self.items():
-            newcount = count + other[elem]
-            if newcount > 0:
-                result[elem] = newcount
-        for elem, count in other.items():
-            if elem not in self and count > 0:
-                result[elem] = count
+        result = Counter(self)
+        result += other
         return result
 
     def __sub__(self, other):
@@ -688,8 +682,9 @@ class Counter(dict):
         if not isinstance(other, Counter):
             return NotImplemented
         result = Counter()
+        other_get = other.get
         for elem, count in self.items():
-            newcount = count - other[elem]
+            newcount = count - other_get(elem, 0)
             if newcount > 0:
                 result[elem] = newcount
         for elem, count in other.items():
@@ -706,15 +701,8 @@ class Counter(dict):
         '''
         if not isinstance(other, Counter):
             return NotImplemented
-        result = Counter()
-        for elem, count in self.items():
-            other_count = other[elem]
-            newcount = other_count if count < other_count else count
-            if newcount > 0:
-                result[elem] = newcount
-        for elem, count in other.items():
-            if elem not in self and count > 0:
-                result[elem] = count
+        result = Counter(self)
+        result |= other
         return result
 
     def __and__(self, other):
@@ -727,8 +715,9 @@ class Counter(dict):
         if not isinstance(other, Counter):
             return NotImplemented
         result = Counter()
+        other_get = other.get
         for elem, count in self.items():
-            other_count = other[elem]
+            other_count = other_get(elem, 0)
             newcount = count if count < other_count else other_count
             if newcount > 0:
                 result[elem] = newcount
@@ -736,20 +725,36 @@ class Counter(dict):
 
     def __pos__(self):
         'Adds an empty counter, effectively stripping negative and zero counts'
-        return self + Counter()
+        result = Counter(self)
+        result._keep_positive()
+        return result
 
     def __neg__(self):
         '''Subtracts from an empty counter.  Strips positive and zero counts,
         and flips the sign on negative counts.
 
         '''
-        return Counter() - self
+        result = Counter()
+        for elem, count in self.items():
+            if count < 0:
+                result[elem] = 0 - count
+        return result
 
     def _keep_positive(self):
         '''Internal method to strip elements with a negative or zero count'''
         nonpositive = [elem for elem, count in self.items() if not count > 0]
-        for elem in nonpositive:
-            del self[elem]
+        if 2*len(nonpositive) < len(self):
+            # If most counts is positive (common case), it is faster to
+            # remove non-positive counts.
+            for elem in nonpositive:
+                del self[elem]
+        else:
+            # If most counts is non-positive, it is faster to create a dict
+            # with positive counts from scratch.
+            del nonpositive
+            positive = {elem: count for elem, count in self.items() if count > 0}
+            self.clear()
+            self.update(positive)
         return self
 
     def __iadd__(self, other):
@@ -761,8 +766,9 @@ class Counter(dict):
         Counter({'b': 4, 'c': 2, 'a': 1})
 
         '''
+        self_get = self.get
         for elem, count in other.items():
-            self[elem] += count
+            self[elem] = self_get(elem, 0) + count
         return self._keep_positive()
 
     def __isub__(self, other):
@@ -774,8 +780,9 @@ class Counter(dict):
         Counter({'b': 2, 'a': 1})
 
         '''
+        self_get = self.get
         for elem, count in other.items():
-            self[elem] -= count
+            self[elem] = self_get(elem, 0) - count
         return self._keep_positive()
 
     def __ior__(self, other):
@@ -787,8 +794,9 @@ class Counter(dict):
         Counter({'b': 3, 'c': 2, 'a': 1})
 
         '''
+        self_get = self.get
         for elem, other_count in other.items():
-            count = self[elem]
+            count = self_get(elem, 0)
             if other_count > count:
                 self[elem] = other_count
         return self._keep_positive()
@@ -802,8 +810,9 @@ class Counter(dict):
         Counter({'b': 1})
 
         '''
+        other_get = other.get
         for elem, count in self.items():
-            other_count = other[elem]
+            other_count = other_get(elem, 0)
             if other_count < count:
                 self[elem] = other_count
         return self._keep_positive()