Hey!
I published an article on how to speed up your test suite (5 times in case of my Capybara tests) by moving your database to tmpfs. I think the technique should work for all tests (MiniTest and RSpec included) that use a database cleaner.
https://www.gregnavis.com/articles/how-to-tune-your-database-to-make-tests-faster.html
Best regards
Greg Navis