cherry-pick very slow on big repository

Hi!

On a big repository (57000 files, 2,5 gigabytes in .git/objects), git cherry-pick is very slow for me (v2.15.0). This is cherry-picking a one-file change, where the file is in the same place on both branches, and which applies cleanly (I am backporting a few fixes to a maintenance version):

$ time git cherry-pick -x 717eb328940ca2e33f14ed27576e656327854b7b
[redacted 391454f16d] Redacted
  Author: Redacted <redacted>
  Date: Mon Oct 16 15:58:05 2017 +0200
  1 file changed, 2 insertions(+), 2 deletions(-)

real 6m9,054s user 5m49,432s sys 0m2,292s

Something is not how it should be here. The repo shares objects (.git/objects/info/alternates) with another repository (I have run "git gc" on both repositories).

Running strace, it seems like it is doing lstat(), open(), mmap(), close() and munmap() on every single file in the repository, which takes a lot of time.

I thought it was just updating the status, but "git status" returns immediately, while cherry-picking takes several minutes for every cherry-pick I do.