python
diff --git a/asyncio/coroutines.py b/asyncio/coroutines.py
@@ -33,12 +33,16 @@
 
 try:
     _types_coroutine = types.coroutine
+    _types_CoroutineType = types.CoroutineType
 except AttributeError:
+    # Python 3.4
     _types_coroutine = None
+    _types_CoroutineType = None
 
 try:
     _inspect_iscoroutinefunction = inspect.iscoroutinefunction
 except AttributeError:
+    # Python 3.4
     _inspect_iscoroutinefunction = lambda func: False
 
 try:
@@ -238,19 +242,27 @@ def wrapper(*args, **kwds):
             w.__qualname__ = getattr(func, '__qualname__', None)
             return w
 
-    wrapper._is_coroutine = True  # For iscoroutinefunction().
+    wrapper._is_coroutine = _is_coroutine  # For iscoroutinefunction().
     return wrapper
 
 
+# A marker for iscoroutinefunction.
+_is_coroutine = object()
+
+
 def iscoroutinefunction(func):
     """Return True if func is a decorated coroutine function."""
-    return (getattr(func, '_is_coroutine', False) or
+    return (getattr(func, '_is_coroutine', None) is _is_coroutine or
             _inspect_iscoroutinefunction(func))
 
 
 _COROUTINE_TYPES = (types.GeneratorType, CoroWrapper)
 if _CoroutineABC is not None:
     _COROUTINE_TYPES += (_CoroutineABC,)
+if _types_CoroutineType is not None:
+    # Prioritize native coroutine check to speed-up
+    # asyncio.iscoroutine.
+    _COROUTINE_TYPES = (_types_CoroutineType,) + _COROUTINE_TYPES
 
 
 def iscoroutine(obj):

diff --git a/tests/test_tasks.py b/tests/test_tasks.py
@@ -1376,6 +1376,8 @@ def fn2():
             yield
         self.assertTrue(asyncio.iscoroutinefunction(fn2))
 
+        self.assertFalse(asyncio.iscoroutinefunction(mock.Mock()))
+
     def test_yield_vs_yield_from(self):
         fut = asyncio.Future(loop=self.loop)