I find WSL to be pretty amazing. I can compile and test Linux binaries on my Windows laptop, and I can even directly use the same directory of source code as I use to build Windows binaries. I haven't noticed a difference in compilation or test times, but I wouldn't expect to unless it was at least an order of magnitude bigger. It's not like I sit there waiting for it to build (normally).

Of course it's absolutely fair if you find WSL poor for your use case. But performance is not really the main thing that makes it "usable for developers", IMO.