I think if you experience the same issue on different distros and both X and wayland, that there is something wrong with the Mesa Driver or CS/steam-proton. Your best change is to write an issue/ask on their forums/githubs pages for help since they know at best what to do.
Since this is not nixos related this is not the right forum for that.
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So, there was an issue with MangoHUD not able being able to grab the data for VRAM and other informations. Which is fixed in their latest releases which is 0.8.1. But in nixpkgs it’s 0.7.2. Maybe try contributing by yourself to the nixpkgs GitHub repository.
The problem with performance, it’s nothing much to do with NixOS particularly. But, linux as a whole the problem with Intel Arc Graphics. The team of Intel trying to make it more reliable but it’s not there yet. If you have information that can help them (intel devs) to do some improvements feels free to post on their community, because I don’t know if they have any GitHub repository where you can create an issue. Oh, by the way, I think you do not need to tinker with your configuration.nix to get some extra FPS. Special, with those hardware.graphics.extraPackages.