A good chunk of people are having the same problem where there is apparently a bug in the mod that causes people to have extreme frame rate drops in certain servers. The most common answer to this is to keep changing servers until you find one that does not have this problem.
I was having the same problems and asked a few people as well and apparently the servers have a bug that causes people to have extreme FPS drops and they are trying to fix it for the next update. The most common answer is to just keep changing servers until you find a decent one that does not have the drop in FPS.
I have the game on as low as all of the graphics can be put on yet it still maxes out at 15 FPS. It confuses me because I can play games like counter-strike and killing floor with 30+ FPS yet arma 2 in general cant run barely at all with very low settings. Any suggestions?