I was playing the MW2 multiplayer. After the installation, the first start on Linux ran without a problem. Sometimes, it has some fps drops for the first several seconds, but after that no more.

Then, I set every graphics to max and searched for a game. The first lobby was ahacked lobby. I was searching for TDM and get Domination (the good old MW2 times :D). Even though it was a hacked lobby, the game ran smoothly on 91 fps. No lag, missing textures or fps drops. It works like out-of-the-box.

Additional info: My mouse is a Logitech G402 with some extra mouse buttons, while the buttons on the left side represent 'back' (Mouse 4) and 'forth' (Mouse 5). These buttons were recognized without any tweaks in Linux or Proton. Edit a control key in the game settings and press the button. BAM. Set. Only the Logitech G-Keys were not regnozied, neither by the game nor by the OS.