Originally was using the latest Proton version with no issues, but there appears to have been a regression recently at the time of writing. Now, after an inconsistent amount of time (sometimes immediately after entering a mission after launching the game, other times as long as 30 minutes after launch), game performance will begin to gradually deteriorate, dramatically reducing frame rate.

This affects the speed of the game and control of the camera, and will persist until the game process is terminated.

Proton version 7.0-4 does not have this issue, nor any other noticable faults.