Earlier quoted context omitted.
I think I'm one of the last people using WSL1. I think everyone jumped to WSL2 because 2 is bigger than 1, but they're really different things. WSL1 lets me keep my files on the Windows side, let's my IDE (PHP storm) remain performant with no Linux to Windows file system overhead, but I still get to use all the Linux CLI tools. I still pay some overhead for that, but it's better than doing it the other way around and…
Wait what? I'm not a Windows user, so I haven't used any version of WSL, but WSL 2 doesn't store the files on the Windows side? It seems to me that the whole "easy access to your Windows files in a Linux environment" was the point of WSL.
WSL2 uses "disks" emulated via image files that are stored as ordinary Windows files (but in a secret place). This makes most file I/O from inside WSL2 faster than from inside WSL1.
WSL2 can use the Windows file system (and vice versa) via file servers.
When a Linux app inside WSL2 wants to use a Windows file, the Linux kernel (there is only ever one for all running WSL2 images!) talks to a Windows app that does the file serving.
When a Windows app wants to use a Linux file from a WSL2 image, the image first has to run (might take a little while for it to boot) and then a daemon there serves the file to the Windows app.
I think one of the file servers use a version of the 9P protocol. Maybe they both do.
https://en.wikipedia.org/wiki/9P_(protocol)
Things that do lots of file I/O (such as compiling) are way faster under WSL2 using local file systems than under WSL1.