You've likely noticed a lot of performance issues in Arma 3 right now if you dont have either:
A. A high end GPU
OR More importantly...
B. A high end CPU
A high end GPU can help with running Arma 3, I'd say the lowest you should go for if you want decent fps is a 7850/650Ti.
A medium-high end CPU is where it's at if you want to get Arma 3 up and running at high fps, regardless of your graphical settings, I'd say if you want to run this game @ Max settings you're going to need a 8350/i5 3570k
Tweaks to help you get where you want:
If you have anything above a 660Ti you should notice that increasing your Graphics settings has little to no effect on the FPS you're getting, Either you go out and you get a new CPU that fits into your current motherboard's socket (AMD: FM2, AM3, or AM3+ Is the most likely case, Intel: socket 1155, or less likely 2011) But right now I cant get a better CPU, and if you're reading this you likely cant either.
1. Depending on your current GPU adjust the graphics settings accordingly, You will get better FPS when you increase the settings to a higher level, try messing around with the Terrain and Texture qualities until you get the best fps in game.
2. Reduce your Resolution, or if you dont want to do that, Reduce the Resolution that it's Rendered at, it is standardly set to whatever your screen resolution is, but you can reduce it down to increase performance.
3. (The most effective) Reduce your Overall Draw Distance, View Distance, Object Distance, Shadow Distance, whatever, reduce it down to ~600, this will increase your performance by a decent margin.
4. Reduce all settings to lowest (This one doesnt do much for me, in fact it actually harms my performance.)
Arma 2/3 Is an extremely CPU reliant game, it uses your CPU to render almost everything that is happening in your game, the GPU is relied on very little, no matter how beautiful it looks @ Ultra, You can expect Performance to be optomized the farther we get into the Alpha, and hopefully will run fantastically upon it's full release (much like Crysis 3 or Planetside 2 was during Beta.)