Re: Poor performance of git describe in big repos

The culprit, according to some callgrind investigation, is
lookup_commit_reference_gently() [for the unannotated case] or
deref_tag() [annotated case] calling parse_object().

Using the scenario you described earlier, I think it ends-up spending
most of its time in check_sha1_signature (both deref_tag and
lookup_commit_reference_gently() go there) with 20% inflating, 80% in
SHA1_Update(). Not much we can do about that, can we ?