What does fast CI even mean? CI is an overgrown shell script running your build and telling you when it fails. In general build tooling has gotten so slow that the cost of whatever CI runner relative to it should be nil. If you want fast CI you need fast tsc, clang, rustc, etc... not a faster program that calls exec on them. A bit more on topic, if you're selling CI and your business fails it's because people you're…
The only part of the Earthly pitch that resonated with me was the ability to run the CI locally. When debugging your CI, it is just faster to find problems when you can run on your own box. I have a gnarly GitHub Actions script that took forever to get right because the debug cycle was 10 minutes long.
But there have been many times when debugging a CI has been necessary, having to figure out what has gone wrong on a remote system is often an issue...ofc often the remote system is itself the problem...