Debugging git-commit slowness on a large repo

Hi,
I have a git repo with about 300k commits,  150k files totaling maybe 7GB.
 Locally committing a small change - say touching fewer than 300 bytes
across 4 files - consistently takes over one second, which seems kinda
slow.  This is using git 1.7.7.4 on a linux 2.6 box.  The time does not
improve after doing a git-gc (my .git dir has maybe 250 files after a git
gc).  The same size commit on a brand new repo takes < 10ms.  Any thoughts
on why committing a small change seems to take a long time on larger repos?

Fwiw, I also tried doing the same test using libgit2 (via the pygit2
wrapper), and it was ever slower (about 6 seconds to commit the same small
change).

Thanks for any thoughts or places to look.

Cheers,
Josh