If you want to run 1 big/slow test file, use Parallel-Split-Test, it splits it into multiple chunks and then runs them in parallel.

PS: If you have lots of tests you should take a look at Parallel-Tests, which runs multiple files in parallel.

This became necessary because in our current project: spec runtime=3h (without integration) / slowest file=5 minutes. Now this time is only 1/3rd.