Same here, though I went to Linux first for several years. WSL file speeds, especially when running npm install, were the impetus that ultimately got me to switch off of Windows.
Either you run npm install from Windows if you are operating on the Windows file system or you run it on WSL if you are operating on the WSL file system both cases will be very fast
Sounds like you were just doing it wrong
Either you run npm install from Windows if you are operating on the Windows file system or you run it on WSL if you are operating on the WSL file system both cases will be very fast