Is there a performance penalty in using a mapped share from the same machine instead of the local filesystem?

On Windows XP SP3, the performance difference was very large. The git log command was 4x slower in my benchmarks.

On Windows 10 (20H2), I see a much more manageable 15% performance penalty.

I would still recommend using the subst command instead:

subst z: c:\some\longer\path

Assuming any fairly recent version of Windows, you shouldn't see any performance difference; modern versions of Windows are generally smart enough to short-circuit unnecessary trips to the network.

Of course, you could always benchmark it and see for sure.