Performance regression in reindex.Fillna.time_float_32, etc.

[TomAugspurger]

Possible regression in https://pandas.pydata.org/speed/pandas/#reindex.Fillna.time_float_32?python=3.6&Cython=&p-method='backfill'&commits=213822a2 and a few others

points to #33271

cc @ShaharNaveh , @WillAyd