Running the complete test suite can be really long. You can running it on multi-fork system with the wonderfull gem of @tmm1, test-queue
If you want do it, you need install in first the gem 'test-queue'
gem install test-queue
After you just need launch the script with adapting runner of mongoid.
./script/rspec-queue-mongoid.rb spec
In my case, the complete test suite down to 2min after a 16min long before.
Some acceptance test use phantomjs to interpret the Javascript in page. To avoid this test you can launch your test by skipping js tag
bundle exec rspec spec --tag="~js"