But they don't really need to fix it. Sure, it'd be nice, but they're targeting developer machines and utilities with WSL, not a server replacement of Linux. Nobody would buy a Windows license just to serve from LAMP stacks on WSL over Azure or something. Speed requirements for dev machines are a little less stringent, and as long as they are hitting better-than-Docker numbers, they will still be providing value.
Sidenote: looks like I/O performance is really not that bad in most cases already, and sometimes even faster than Linux distros like Ubuntu: https://www.phoronix.com/scan.php?page=article&item=wsl-febr...