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r58099 added an exception to the common case of PyObject_IsInstance(),
when the class has no __instancecheck__ attribute. This makes
isinstance(3, int) take 4x as long as in python 2.5.

I'll set this to critical to ensure that I look at it at least once
before we release. I'm not sure however that we can do much about it --
nor that it matters much in practice.
Perhaps we could speed up certain common isinstance() calls by skipping
the lookup for non-heap types; I believe those never override
__instancheck__.

Would it help to implement a default __instancecheck__ and
__subclasscheck__ for object (or for type), that subclasses can override?

Perhaps, though I'm not sure if that doesn't slow things down further
due to the complicated protocol for calling it. Also, there's a
recursion check in the built-in implementation.

The attribute lookup cost can mostly be eliminated if __instancecheck__
were given a tp slot.

Yeah, but tp_ slots are expensive themselves (mostly in the amount of
code that needs to be changed -- see typeobject.c).

No doubt it would take some work. IMO, code for a slot is worth it;
otherwise, many apps will have to pay the price for ABCs even if they
don't use the feature. For my company, that would deter an upgrade to 2.6.

Attaching a small patch to speed-up one easy case.

Issue #2534 has a patch which speeds up isinstance and issubclass by
implementing type.__instancecheck__ and type.__subclasscheck__.

Does anyone care about this still?
I added some comments on
http://codereview.appspot.com/504 

% ./python.exe -mtimeit 'isinstance(3, int)'
1000000 loops, best of 3: 0.269 usec per loop
% ../release25-maint/python.exe -mtimeit 'isinstance(3, int)'1000000
loops, best of 3: 0.335 usec per loop
So I'd say its no longer 4x slower these days.
Looking at the Object/abstract.c it appears the fix to this has already
been checked in. PyObject_IsInstance already has this in it:
 /* Quick test for an exact match */
 if (Py_TYPE(inst) == (PyTypeObject *)cls)
 return 1;
closing.