This is a game that uses the source engine. Source is great at alot of things but I think this game pushes it to its limit in terms of environment size and enemy count. If you feel the game stutter it's probably because it just loaded something in. This happens on the windows version too so it's not a linux specific gripe. Lowering the in-game settings does mitigate this but it doesn't completely stop it from happening. Maybe these issues don't appear on better hardware but on that I can't say for sure. Just bear in mind that it's not proton that is making the game janky. It's just how the game is.