Bundler version 1.5.1 allows gems to be installed in parallel. The Ruby buildpack now uses Bundler 1.5.1 and installs using 4 parallel processes using the flag
Installing Ruby Gems can take a long amount of time due to the number of network requests that are required. By making these requests in parallel we can better utilize the resources available and greatly speed up deploy time.
In your Ruby deploy output you should now see the flag
Running: bundle install --without development:test --path vendor/bundle --binstubs vendor/bundle/bin -j4 --deployment
This change has been reverted due to potential problems installing self referential gems. When the issue has been fixed in bundler, the feature will be re-redeployed.