Feature request: secondary index by path fragment

Maybe this already happens in the code, but if not, please consider it as a feature request.

Gentoo has some tooling that boils down to repeated runs of 'git log -- somepath/' via cgit as well as other shell tooling.

If the path is relatively deep for the tree (e.g. to a specific file or sub-directory), the size of history [1] makes that a very slow operation to go all the way back to the initial repo commit: ~12 seconds per operation on fast hardware, ~45 seconds on slower harder - even with the packs cached.

I was wondering if Git could gain a secondary index of commits, based on path prefixes, that would speed up the 'git log' run.

It would need to be fast to append to the secondary index, because Gentoo gets a steady flow of commits 24/7.

[1] 825k+ commits based on GitHub stats.