jeblair was kind enough to help me debug my problem with slow “git review” uploads for Openstack projects just now. It turns out that part of my standard configuration for ssh is to enable ControlMaster and ControlPersist. I mostly do this because the machines I use at Canonical are a very long way away from my home in Australia, and its nice to have slightly faster connections when you ssh to a machine. However, gerrit is incompatible with these options as best as we can tell.
So, if your git reviews are taking 10 to 20 minutes to upload like mine were, check that you’re not using persistent connections. Excluding review.openstack.org from that part of my configuration has made a massive difference to the speed of uploads for me.