jes out of all people to be familiar with this notorious problem, I figured you guys would =p. Arma 3 has always had FPS problems, although I heard beta had resolved some issues. Nonetheless, high quality computers such as mine ^.^ can only get about 20 FPS out of ultra in multiplayer, while some lame ass computers can run on very high with like 40 FPS. It's not your computer....its Arma. Arma 2 and 3 has always been very CPU intensive. Your best bet is to hook up a CPU that doesn't necessarily have a good amount of cores, but runs at a high speed.
Also speaking from a personal perspective, I've noticed there is a relationship between the amount of players and my fps that seems too strong...for like every 3 players I have on the server with me I lose 1-2 FPS. Hope this helped a bit ^.^