I wonder why larger companies don't use self hosted github runners. You can buy a pretty beefy machine (TB of RAM, 256 cores, fast NVMe disk) and tests will run faster than on any hosted platform. Plus you don't have to shard as aggressively because more fits into one machine, benefit of shared page cache, shared persistent disk, can easily preserve working directory (for example my pnpm install takes 0 seconds, because the node_modules folder is already present from the previous run).