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.
That is what I meant by the BEST FREE CI provider. It is not the best if you have money for something better.
I built a similar matrix to test wazero (of which I'm a maintainer).
But if you do know of alternatives, I'm all ears.