totally agree, github actions has done an excellent job at this lowest layer of the build pipeline today but is woefully inadequate the minute your org hits north of 50 engineers
Yeah that still holds true to some extent today with the GHA cache. Blacksmith colocates its cache with our CI runners, and ensures that they're in the same local network allowing us to saturate the NIC and provide much faster cache reads/writes. We're also thinking of clever ways to avoid downloading from a cache entirely and instead bind mount cache volumes over the network into the CI runner. Still early days, but stay tuned!
This is exactly the sort of insight that led us to work on Blacksmith. Since we own the hardware we run CI jobs on there are some exciting things we can do to make these "sticky disks" work the way you describe it. Stay tuned!