PERF: find_stack_level

[jbrockmendel]

Profiling pandas/tests/groupby/test_function.py::test_builtins_apply I'm seeing 18.78 seconds spent in groupby.apply, with 14.06 of that spent in 1732 calls of find_stack_level.

Maybe we can make find_stack_level lazy so it is only called when the warning is actually issued?

[jreback]

yeah or could have a

show_warning method that only is executed in the warnjng block itself

eg

if :
show_warning(,,,)