I'm only half-awake, so maybe I'm missing something obvious. Why is the README talking about a git commit hook? Why not just take the data from the log?
It seems like for performance reasons, git-stats wants to update the calendar on commit instead of generating it on the fly. git-cal[0] just does this on the fly and works reasonably fast for me. The UI looks very comparable. [0] https://github.com/k4rthik/git-cal
Hmm, just installed git-cal and it certainly is adequately fast on my main work repo. But it seems to just be for the current branch? (That seems sensible enough, but also makes it somewhat less useful.)