reads and writes transparently redirect to the equivalent of a tmpfs, up to a certain size

The last bit (emphasis added) sounds novel to me, I don't think I've heard before of anybody doing that. It sounds like an almost-"free" way to get a ton of performance ("almost" because somebody has to figure out the sizing. Though, I bet you could automate that by having your tool export a "desired size" metric that's equal to the high watermark of tmpfs-like storage used during the CI run)