If it's like anything else running in WSL, absolutely as slow as balls. Most Linux apps are written with the assumption that filesystem operations are incredibly fast, whereas that's not true with Windows. Most games open one big file and do big reads from it so it's not such a problem, whereas something like Git assumes that touching tens of thousands of files should be basically instant.

Linux can be installed on any computer for free, so I wouldn't call not making a Windows binary exclusive, unless all the games I need Proton to run are exclusive too.

If Windows users want to play Half Life 3 they can just ask Microsoft to make a reverse WINE lol

Of course they don't but it would be funny to see if Windows users spend their time with compatibility layers just this once. You know, for the sake of chaotic good.