DES: How much perf penalty will we accept to get rid of libreduction?
xref some discussion https://github.com/pandas-dev/pandas/pull/40171#issuecomment-790219422 

libreduction and the associated callers are a disproportionate maintenance headache [citation needed].  It would be nice to be able to rip it out and just have one path for those methods, but that would entail a non-trivial performance hit.  Recently, though, we've managed to optimize the pure-python path a bit, and im optimistic we can shave off some more of the difference.

The question: how much of a perf penalty are we willing to accept in order to remove libreduction?

Copying from https://github.com/pandas-dev/pandas/pull/40171#issuecomment-789077389

> I'll throw out a number: if we could get worst-case down to within about 3x and most-cases within 1.5x, I'd be open to removing the cython paths. They are a disproportionate producer of headaches. (From #36459 (possibly out of date) "entirely disabling the cython path leads to 4 currently-xfailed tests passing")

> Besides which, if/when cython3 becomes available, we may have to get rid of these anyway.