A lot of people were having frame issues, even with good computers, before Beta because the game wasn't fully optimized for everything yet. I can run the game on a lot of frames now since my Graphics Card was optimized for Arma 3 and it fixed it.
---------- Post added at 07:53 AM ---------- Previous post was at 07:51 AM ----------
Using Auto-Detect for your graphics in Arma 3 is pretty helpful, it will usually go above a little and it lags, but then you can just set it a little lower to get the perfect graphics.
Ah, well either way, you're going to lag because
A: Only a 'decent' CPU
B: Massive loads on CPU because you have a Radeon GPU
C: Why B? Because of PhysX, that's why.
Trying to play arma3 with those specs is a fail.