It seems the bulk of the time was from installing ruby gems for Jekyll. Maybe GitHub Actions mirrors them or something so it can run faster?
Besides that, it works great. Actual live performance seems as snappy as one would expect from a static site and setting it up was almost one click. I'll definitely be looking into this in more detail.
First impressions are good and I may well migrate over for static site and API server (API server is currently running on a droplet as a single docker container, so seems like a nice way to lower the effort I have to put in). Everything else is running on AWS for.. reasons.. so this looks like a nice way to simplify my non-AWS stuff. I'll be experimenting with it over the coming days! The bandwidth limits are the biggest concern.