In [1]: import timeit

In [4]: class A(object):
   ...:     pass
   ...:

In [5]: item = object()

In [6]: item_a = A()

In [7]: def descendent_isinstance():
   ...:     return isinstance(item_a, object)
   ...:

In [8]: def direct_isinstance():
   ...:     return isinstance(item_a, A)
   ...:

In [9]: def direct_isinstance_builtin():
   ...:     return isinstance(item, object)
   ...:

In [10]: def not_isinstance():
    ...:     return isinstance(item, A)
    ...:

In [11]: descendent_isinstance()
Out[11]: True

In [12]: direct_isinstance()
Out[12]: True

In [14]: direct_isinstance_builtin()
Out[14]: True

In [15]: not_isinstance()
Out[15]: False

In [17]: tests = {
    ...:     "descendent_isinstance": descendent_isinstance,
    ...:     "direct_isinstance": direct_isinstance,
    ...:     "direct_isinstance_builtin": direct_isinstance_builtin,
    ...:     "not_isinstance": not_isinstance,
    ...:     "item": item,
    ...:     "item_a": item_a,
    ...:     "A": A,
    ...: }

In [18]: timeit.repeat(stmt="descendent_isinstance()", globals=tests)
Out[18]:
[0.18606656999997995,
 0.17976311300003545,
 0.17793933600000855,
 0.17901940199999444,
 0.1791184380000459]

In [19]: timeit.repeat(stmt="direct_isinstance()", globals=tests)
Out[19]:
[0.16835473799994816,
 0.16796777699994436,
 0.16732346500009498,
 0.1671851889999516,
 0.1646581120000974]

In [20]: timeit.repeat(stmt="direct_isinstance_builtin()", globals=tests)
Out[20]:
[0.17491985099991325,
 0.17345420999993166,
 0.17239584300000388,
 0.1734395940000013,
 0.17397786800006543]

In [21]: timeit.repeat(stmt="not_isinstance()", globals=tests)
Out[21]:
[0.19989036399999804,
 0.1967721519999941,
 0.1962455799999816,
 0.19687672300005943,
 0.19701133199998822]