Performance regression in git fetch between 1.8.3.4 and 1.8.5.3

At work, we recently upgraded our git version from 1.8.3.4 to 1.8.5.3. We've noticed a significant performance regression in git fetch. The numbers below are for an up-to-date branch (that is, no data is actually being fetched) for a git-over-ssh remote on our gitorious server.

This is an auxiliary repository, so its size is probably between 6-7 GiB. It does not have an especially large number of refs, but is a clone of our main repository.

Other than the new version, the only difference is that 1.8.5.3 is built against libpcre. These are both running on the same CentOS 6 system; literally the only difference is installing one RPM or the other.

1.8.3.4:

  brianc ok # for i in `seq 1 3`; do time git fetch; done
  git fetch  0.86s user 0.18s system 78% cpu 1.314 total
  git fetch  0.88s user 0.19s system 79% cpu 1.348 total
  git fetch  0.84s user 0.18s system 78% cpu 1.299 total

1.8.5.3:

  brianc ok # for i in `seq 1 3`; do time git fetch; done
  git fetch  17.11s user 1.22s system 98% cpu 18.652 total
  git fetch  16.74s user 1.24s system 98% cpu 18.286 total
  git fetch  17.78s user 1.35s system 98% cpu 19.446 total

Does anyone have any idea what might be causing this?