Gave a shot to the new Bundler 1.4.0RC1 during the weekend and found out it now supports gem installation in parallel. To invoke you need to pass --jobs parameter with number of threads you want it to run – for me the best performance was achieved by specifying the number physical CPU cores.
I've tested on the AppFab Show archive.org snapshot app. In my case (CPU with 2-core i7 with HT) the speedup was 50%. I've also tried with a number greater than the number of physical cores but the performance was 15%-20% worse.
Bundler 1.3.5
➜ appfab git:(master) bundle --version
Bundler version 1.3.5
➜ appfab git:(master) time bundle install --path=vendor/
483.44 real 126.23 user 39.96 sys
Bundler 1.4.0RC1
gem update bundler —pre
➜ appfab git:(master) bundle --version
Bundler version 1.4.0.rc.1
➜ appfab git:(master) time bundle install --jobs 2 --path=vendor/
234.65 real 136.37 user 42.40 sys
Happy coding.