PERF: regression in DataFrame.shift method performance
Performance regression in `shift` caused by #40536 

I don't know if it's a particularly useful benchmark, but there is a big slowdown potentially related to this PR: https://pandas.pydata.org/speed/pandas/#frame_methods.Shift.time_shift?python=3.8&Cython=0.29.21&p-axis=1&commits=05a0e98a-bc62e768

_Originally posted by @jorisvandenbossche in https://github.com/pandas-dev/pandas/issues/40536#issuecomment-808265872_