This is great. I do this sort of git-blame accounting to track how much code is written by AI versus humans in each release of my app.
My "blame script" has been slowing down as the repo size increases. I was just about to add caching, like you have.
Have you thought about adding the ability to limit the stats based on a set of file patterns? Perhaps like this, where the file follows gitignore conventions?
git-who table -include-file <fname>
git-who table -ignore-file <fname>
I tried to quickly add this functionality but unfortunately I don't know go.