very slow cherry-pick'ing (old-2.6-bkcvs tree)

Our group maintains Unionfs on the latest -rc kernel, but we also maintain
several backports going all the way to 2.6.9.  Once we complete the
development and testing of a feature/fix in -latest, we cherry-pick those
commits to older backports, and test those.  When I cherry-pick from -latest
to my 2.6.{22,21,20,19,18} repositories, it works reasonably fast.  But when
I cherry-pick to my 2.6.9 tree, it runs about 20 times slower!  Why?  Is
there anything I can do to inspect what's going on and perhaps speed up the
cherry-picking process?

Some info:

My 2.6.{18,19,20,21,22} trees were cloned from

   git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-2.6.<N>.y.git

My 2.6.9 tree, however, was cloned from

   git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/old-2.6-bkcvs.git

after which, I truncated the tree (git-reset) to Linus commit which read
"Linux 2.6.9-final".

BTW, I first git-clone --bare to my git server, and then I clone from it to
my test machine.

BTW2, I do git-repack and prune-packed every week.

Thanks,
Erez.