In terms of machine time? I'd guess so, but I'd also guess calling it for each of `git ls-files` is not the fastest way to get the most recent commit of all the files in a typical scenario (large repo, recent time of interest, most files not changed that recently). And especially if you're okay assuming the most recent commit by parentage is the one you want (even if parentage doesn't match chronological order); then filtering with `git log --name-only --since` (no path argument) seems better. A `git log` post-processing script could also stop early if it's seen a commit for each the files of interest (again, assuming you don't want to return a later date that is attached to an ancestor commit).
Anyway, cool tool, and it's rare that I would actually care about the machine efficiency of this pipeline enough to bother with the approach I just described.