I think I missed this when I looked first time around. I don't think it's safe to install plugins in parallel though. The install logic assumes that no one else is touching files at the same time. If it really gives a big speed-up, then we could look at adding in the concept of "locks" on files so that it is safe to run in parallel, but I think this would be best made in a separate change (since caching the xcode project file is a big boost just on its own)