I believe that is a bug on the Gitorious side of things. Gitorious loads the entire commit history of fgdata when you want to do a merge request. Since FlightGear went through quite some developments over the past years, this is an enormous blob of data to load. Usually freezing my browser. It does seem to help a bit, if you git pull&push to sync your fgdata clone before pushing the merge-request commits.
I've mentioned this at the Gitorious IRC channel and one of the Gitorious-devs told me that they're working on a revamp of the system. Hopefully including a better way of merge requests. However, apparently some people seem to have little or no problems at all. Would be interesting to know what they do different (if they do)...