List last commit dates for a large number of files, quickly

Try this.

In git, each commit references a tree object which has pointers to the state of each file (the files being blob objects).

So, what you want to do is write a program which starts out with a list of all the files in which you're interested, and begins at the HEAD object (SHA1 commit obtained via git rev-parse HEAD). It checks to see if any of the "files of interest" are modified in that tree (tree gotten from "tree" attribute of git cat-file commit [SHA1]) - note, you'll have to descend to the subtrees for each directory. If they are modified (meaning a different SHA1 hash from the one they had in the "previous" revision), it removes each such from the interest set and prints the appropriate information. Then it continues to each parent of the current tree. This continues until the set-of-interest is empty.

If you want the maximal speed, you'll use the git C API. If you don't want that much speed, you can use git cat-file tree [SHA1 hash] (or, easier, git ls-tree [SHA1 hash] [files]), which is going to perform the absolute minimal amount of work to read a particular tree object (it's part of the plumbing layer).

It's questionable how well this will continue to work in the future, but if forward-compat is a bigger issue you can move up a level from git cat-file - but as you already discovered, git log is comparatively slow as it's part of the porcelain, not the plumbing.

See here for a pretty good resource on how git's object model works.