One thing that really sped up our test suite was by creating an NGINX proxy that served up all the static files instead of making rails do it. This saved us about 10 minutes off our 30 minute tests.
test-queue supports minitest too, and follows the same basic design outlined in this article: a central queue sorted by slowest tests first, with test runners forked off either locally or on other machines to consume off the queue.
We use test-queue in a dozen different projects at GitHub and most CI runs finish in ~30s. The largest suite is for the github/github rails app which runs 30 minutes of tests in 90s.
https://github.com/tmm1/test-queue
One thing that really sped up our test suite was by creating an NGINX proxy that served up all the static files instead of making rails do it. This saved us about 10 minutes off our 30 minute tests.