RMA 3 is the best milsim out there, hands down. But it's a bear to keep FPS up on anything other than singleplayer. Patrol Ops runs at lower FPS than some servers due to the content and the automated and sophisticated nature of the game, however most of the FPS issues are client-side.
While there's lots of tutorials on how to better optimize your ARMA 3 install to better improve client-side performance, I thought I'd share one with you that includes step-by-step instructions (with pictures!) to make this process a little easier for you.