[issue2303] isinstance is 4x as slow as in 2.5 because the common case raises

[Guido van Rossum]

Guido van Rossum <guido at python.org> added the comment:

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__.

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priority:  -> critical

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