First goto you arma2 folder inside your documents and open arma2oa.cfg file and change AToC to 1 or 0 and use FXAA instead of ingame AA. Also with the ingame setting set hdr to low and PP to low or med. Also there really are no performance bugs in the game its mainly the AI since the game renders all the AI in the map so its a huge impact on your cpu since its calculating hundreds of AI throughout the entire map. But thankfully in arma 3 most of that will be fixed.