Feel free to report here any issue you encountered in game.

But first of all, know that low FPS will get you A.I issues, lag hits, and disfunctionnal interactions with environment, so that just means you have to get a lighter version of the mission.

One more thing : lowering graphics wont help, but disabling PIP, lowering clouds, lowering shadows, setting overall and objects view distance between 2000 and 3000 will help your FPS.