WSL 2 is virtualization-based (and likely Microsoft’s primary path going forward), but WSL 1 was not— it actually did implement the Linux ABI on top of the Windows kernel, allowing Linux processes to coexist alongside Windows processes (with no actual Linux kernel involved at any point).
It’s actually a pretty neat architecture— I’m on my phone right now and can’t track down a link, but it’s worth reading about if you’ve got the time. Kind of a shame that they moved on to the virtualization approach, but understandable— they’re trying to solve the same sort of problem as Wine, where you’ve got to mimic all the quirks of a foreign OS and it’s also a moving target (so you’re never “done”).
File system access is super slow on WSL. This was one the drivers. If I recall correctly it is because some common Linux syscalls (stat?) are missing/slow on Windows NT kernels.
The filesystem in general is known to be much slower on Windows due to it's extreme flexibility, but Linux design decisions assumed a much more performant filesystem. Hence why running linux on windows slammed into the performance problem.
It’s actually a pretty neat architecture— I’m on my phone right now and can’t track down a link, but it’s worth reading about if you’ve got the time. Kind of a shame that they moved on to the virtualization approach, but understandable— they’re trying to solve the same sort of problem as Wine, where you’ve got to mimic all the quirks of a foreign OS and it’s also a moving target (so you’re never “done”).