PERF: Regression in groupby ops from adding skipna

[rhshadrach]

pandas-dev/asv-runner#42

Due to #60752 - cc @snitish

[rhshadrach]

Some performance degradation is unavoidable, but some of these seem larger than I had expected. Might be worth a look into. E.g.

https://rhshadrach.github.io/asv-runner/#groupby.GroupByCythonAgg.time_frame_agg?p-dtype=%27float64%27&p-method=%27max%27

[snitish]

Thanks for flagging @rhshadrach. I identified the cause of the performance degradation.

https://github.com/snitish/pandas/blob/0fc49df08fb81233750a3007bc8b5b2cd5b5e675/pandas/_libs/groupby.pyx#L1910-L1916

The isna_result = ... lines can be moved into the if not skipna condition as this flag is only used when skipna is False. That seems to improve the run time quite a bit. Will create a PR with the fix.