ArmA 2 Multiplayer has low FPS and singleplayer has good FPS. HELP!

So if I join a ArmA 2 server like wasteland or warfare or dayz with people, the game gets around 15-25 FPS!? Yet when I play singleplayer on high settings I get 60-100 FPS! Me and my friend play custom CO-OP missions and warfare together and it does not have low FPS. Even when there is a huge battle between AI Vs AI or Me/Friends VS AI. But when I join a server like wasteland or dayz with him and other people. I get really low FPS? Why is this? People say its because there is a lot more going on ( explosions, gun shots, people spawing in, etc. ) but this cant be because i get 60-100 fps on high settings. when i got onto multiplayer with the same mission, 15-25 fps. On HIGH settings, 15-25 FPS...on LOW settings...15-25 FPS:butbut: Please tell me if there is a fix. I have the latest ArmA 2 OA patch. Ill provide my system specs.

My PC Specs are:

Card: SMBX2450(1920x1080@60Hz) ATI Radeon HD 7770 Series

RAM: 4GB Ram

OS: Microsoft Windows 7 Home Premium 64-bit

CPU: AMD Athlon II X4 640

Thanks!

It's because the game is not optimized at all for this kind of 40+ player missions with lots of objects/vehicles/loot, so unless you have a super powerful CPU 2500K or better you're out of luck.