You need to add an exclusion or two to Windows Defender. The yarn folder in %APPDATA% is one (if you're running yarn locally, same deal for npm), and the path to where you keep your repos is the other (which is a lot easier if that doesn't exist entirely inside your WSL filesystem).
The reason it's so slow is because the virus scanner is running on the insane amount of tiny files a typical JS app will have. Used to take me 30 minutes each time until I did that; it was still slower than Linux/Mac but it was coffee-break slow, not three-course meal slow.
The windows subsystem for linux? It's dealing with lots of tiny files going through a virtualization layer, you might want to just use the Windows native version of Yarn which is very fast.
The reason it's so slow is because the virus scanner is running on the insane amount of tiny files a typical JS app will have. Used to take me 30 minutes each time until I did that; it was still slower than Linux/Mac but it was coffee-break slow, not three-course meal slow.