PERF: groupby.value_counts

[rhshadrach]

Ref: 4f42ecb#r102797297

The current implementation of groupby.value_counts uses a lot of the public API for operations. There may be opportunities here to use the pandas internals more directly for performance.

[renatocmaciel]

take

[adirajput2000]

take

[Dukastlik]

Hi, @rhshadrach ! I would like to work on this issue. Can you please be more specific about speeding up groupby.value_counts for sort=True case? As I see - sorting of resulting Series is done after the call to .size(), but we what to change .size method as I understand it.