To precise, you say WSL1 is faster compared to Windows, or compared to WSL2? With WSL2 (and native-comp emacs branch) I've never noticed any unusual slowdowns with magit or other.
WSL1 process creation is faster compared to Windows, because part of the black magic it does to run Linux processes on NT kernel is using minimal processes - so called "pico processes"[0]. These are much leaner than standard Windows processes, and more suited for UNIX-style workflow.
I can't say if it's faster relative to WSL2, but I'd guess so. WSL2 is a full VM, after all.
It shouldn't actually be a noticeable difference. HW virtualization means that unless the guest is doing I/O or needs to be interrupted to yield to the host, the guest is kind of just doing its thing. Spawning a new user space process in a VM should, in theory, be basically the same speed as spawning a new user space process on the bare metal. How that compares to the WSL1 approach of pico processes I don't know, but Linux generally has a very optimized path for spawning a process that I would imagine is competitive.
I can't say if it's faster relative to WSL2, but I'd guess so. WSL2 is a full VM, after all.
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[0] - https://docs.microsoft.com/en-us/archive/blogs/wsl/pico-proc...