The basic classes int, dict, list, tuple ... all have a fast path for Py_*Check(obj):

#define PyLong_Check(op) \
        PyType_FastSubclass(Py_TYPE(op), Py_TPFLAGS_LONG_SUBCLASS)


except for float. I propose to add a Py_TPFLAGS_FLOAT_SUBCLASS enum and use it to implement PyType_FastSubclass for float.

"complex" is also missing a fast path

It is because PyFloat_Check() and PyComplex_Check() are rarely used in comparison with checks for integers, strings, etc. The flags space is limited, so it is better to use it for something more important.

There were reasons for not adding such flags at first place and I think that they are still valid.

Thanks for the suggestion, but I also concur with Serhiy.

New changeset 2ef06d412531d1163dbc72877c88aedf3ed82a25 by Matti Picus in branch 'main':
bpo-46131: add fastpath for PyFloat_Check() (#30200)
https://github.com/python/cpython/commit/2ef06d412531d1163dbc72877c88aedf3ed82a25

What do ya'all think of widen the flags field to int64_t so that there is more room for optimizations like this?

New changeset 29ea68bd1dcf30842c2ed908a6d815bc1d90f484 by Raymond Hettinger in branch 'main':
Revert "bpo-46131: add fastpath for PyFloat_Check() (GH-30200)" (GH-30208)
https://github.com/python/cpython/commit/29ea68bd1dcf30842c2ed908a6d815bc1d90f484

That would not change that slot space is at a premium and that we prefer to use that space for high payoff optimizations.