This explains a lot! I assumed the speed I was getting in windows was normal until I tried fastai in Ubuntu on my system on the exact same notebook, and it ran / trained ~3x faster.
On my hardware, your fix took an unet / resnet101 128x128x3 BS = 8 epoch from around 3:10 to 1:25. I get around :55 in ubuntu on the same hardware, but this is at least more reasonable on windows.
Thanks.