As an example we recently had a Ruby application that had a test suite that was taking literally an hour per build, but turned out it was running entirely seque

As an example we recently had a Ruby application that had a test suite that was taking literally an hour per build, but turned out it was running entirely sequential by default, using only 1 core. I spent an afternoon migrating our CI runners to split the workload across all available cores and now it's 5 minutes per build. And that was just the low hanging fruit, it can be significantly improved further but there's obviously diminishing returns