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With the introduction of ABCs, PyObject_IsInstance (and for this issue, everything is also valid for PyObject_IsSubclass) has to check for a type's __instancecheck__ before falling back to the built-in behavior.
However, the "type" type has an __instancecheck__ method (that calls the built-in behavior too), probably for consistency on the Python side.
This means that the fast path is never taken, and every isinstance() call is slowed down unnecessarily.
This patch introduces a new fast path by checking for PyType_Type exactly before looking up the __instancecheck__.
It also introduces a check for exact match in PyObject_IsSubclass() analogous to one that is already present in PyObject_IsInstance(). Note that this will break __subclasscheck__ implementations that would deny issubclass(C, C).
This patch is not only useful for speeding up isinstance() and issubclass() calls, but also has other effects such as not slowing down the improvement of issue #12029.

Addressing review comments.

New changeset 4f33a4a2b425 by Georg Brandl in branch 'default':
Closes #22540: speed up PyObject_IsInstance and PyObject_IsSubclass in the common case that the second argument has metaclass "type".
https://hg.python.org/cpython/rev/4f33a4a2b425 

A buildbot is failing since your change.
http://buildbot.python.org/all/builders/AMD64%20Debian%20root%203.x/builds/1261/steps/test/logs/stdio
======================================================================
ERROR: test_decimal_fraction_comparison (test.test_decimal.CUsabilityTest)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/test_decimal.py", line 1711, in test_decimal_fraction_comparison
 self.assertLess(D(0), F(1,9999999999999999999999999999999999999))
 File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/unittest/case.py", line 1174, in assertLess
 if not a < b:
TypeError: unorderable types: decimal.Decimal() < Fraction()
======================================================================
FAIL: test_abc (test.test_decimal.CPythonAPItests)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/test_decimal.py", line 2404, in test_abc
 self.assertTrue(issubclass(Decimal, numbers.Number))
AssertionError: False is not true
======================================================================
ERROR: test_mixed_comparisons (test.test_numeric_tower.ComparisonTest)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/test_numeric_tower.py", line 173, in test_mixed_comparisons
 self.assertLess(first, second)
 File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/unittest/case.py", line 1174, in assertLess
 if not a < b:
TypeError: unorderable types: decimal.Decimal() < Fraction()

The test_decimal failure can be reproduced using:
 ./python -m test -W test_datetime test_decimal

New changeset 6cfe929d1de5 by Antoine Pitrou in branch 'default':
Make test_datetime a better citizen (issue #22540)
https://hg.python.org/cpython/rev/6cfe929d1de5 

Thanks for fixing! I did run "make test" locally, but that did not produce the failure.