At causes, our tests run in about 20 minutes on 1 machine, and 4 minutes with buffet and 5.
Why is Buffet better? (I'm genuinely interested)
Alternatively, you have parallel_tests ( https://github.com/grosser/parallel_tests ) which can run on multiple cores. That's great if you use a mac pro, or some behemoth machine.
Little known there's testbot ( https://github.com/joakimk/testbot ) which can run tests across machines & cores. They've even worked in a bootstrapping process https://github.com/joakimk/testbot_cloud
From ( https://github.com/joakimk/testbot/wiki/How-testbot-is-used ) "... 60 minutes of tests to run in 10 minutes using 16 medium EC2 instances (32 cores), read the blog post." reports thoughtbot!
Code softly hackers.