Very small once the shaders compile; for things built in to steam (even those using experimental settings) that's done before the game launches - then you get +- 10% (some things run faster in vulkan, even if not written for it). That's for most games, at least, but YMMV.How big is the performance delta typically for things that DO work these days?
It's been a while since I did any testing on my own, but when I did, there was still a 20+% performance hit in Linux compared to windows (and sometimes that was +++++) even for native ports, let alone using Proton/DXVK.
Has that improved at all?
I still dual boot to Windows simply because I can't afford to lose any performance at all if I want to keep certain 4k titles above 60fps.
For things that compile on the fly (Lutris), it tends to be slightly jerky the very first time the shader is needed, and then the same +- as before. They've gotten VERY good at this, DX12 aside - and this does tend to be a recent development. Star Citizen (before a patch broke it) was about 5% less than windows on teh same hardware. Other things are no different - granted, I'm doing this at 4k on a 3080+10980XE, so YMMV (Neo G9 monitor).
I do dual boot that system - anything I can run in linux though, I do. It's my main "work" workstation.