

The compatibility layer is overhead, but the key difference for many games is that DXVK swaps directX for Vulkan, and Vulkan often gets better performance.
The performance gains of using steamOS are twofold, there’s less OS load (this is particularly noticeable in low performance games, windows will consume much more battery on a game like Dead Cells than SteamOS will), and there’s also a vulkan performance increase for some games. My understanding is if you see a big performance increase in a demanding game, that’s usually thanks to vulkan.
Yes, from what I’ve been told that actually does improve performance in many games.