I've been planning to implement NixOS tests based on virtiofs instead of 9p for a while, but haven't got it working yet (at the time I tried, the available uid mapping strategies didn't cover what was needed for NixOS tests; there's a patch which I haven't got around to trying out yet though.) and this big improvement takes the pain away a bit so my motivation to finish that off is lower. I'll be sure to write about it if I do though!