This reduced launch time of rspec for my rails project in Ubuntu VirtualBox on Win7 host from 35 secs to 2 sec.
sudo gem install spork --prerelease
Or add following to Rails Gemfile:
gem 'spork', '>= 0.9.0.rc'
Run in your Rails project root.
This will add some code and instructions to spec/spec_helper.rb. Follow the instructions and distribute configuration between sections.
Start spork server:
In another terminal launch specs (first green dot should appear 10 times faster the usual):
rspec --drb spec/controllers/your_controller_spec.rb
Read here how to autorestart spork with guard: