And it seems they maintain their own apt mirror or whatever, the package install speed looks unreasonably fast.
But they currently only give 250 minute (default config) per week to free account now, probably due to the runner cost.
They give you 10,000 minutes of build time on their 2cpu/4gb workers per week with a max of 4 concurrent workers.
Edit: Looks like you get 4 concurrent jobs if you register as an open source project.
My biggest complaint has been it lacks an very useful built in ability that Travis had, which is automatically running your builds on the merge of your current branch and target branch. You can right a script to do it yourself, but there missing some environmental variables to do it easily (because the build can start before the target branch is known).
I've also hacked about the 'merge' problem but it's not entirely satisfying.
That all said, with GH actions currently being entirely free, and doing PRs better than CircleCI, we might shift our focus there.