How to efficiently blame an entire repo?

Let's say you've got a repo with ~ 40K files and 35K commits. Well-packed .git is about 800MB.

You want to find out how many lines of code a particular group of individuals has contributed to HEAD.

The naive solution is to run git blame on all 40K files grep'ing for the just the authors you want.

Possibly a step up from that is first using log --name-status --author=... to find just the files which have been touched by those authors and then blaming only those files.

I guess the next step up would be parsing the diff hunks output by log -p, but then you're basically re-implementing blame I think.

Am I missing a clever solution?

j.