The best free CI system in the world has macOS 15 runners running at 75% capacity due to a background process that consumes 100% CPU [1]. The problem is known to them since May but not fixed half a year later.
I would love to know it. I use GH Actions to test my open source code on MacOS, Windows and Linux FOR FREE which is amazing. Without them I just wouldn't test on anything other than what I happen to have locally or what TravisCI/CircleCI had in their free tier before GH Actions made things so much easier.
[1] https://github.com/actions/runner-images/issues/13358