It is your CPU more than likely. You need to have over 2.5Ghz to stand a good chance of getting high quality gfx. 3Ghz+ if you want to start pumping this up to HD resolutions. The PCSX2 hardly uses a Gfx card at all, only to actually render whatever res to the screen, not to compute the actual Gfx like a normal game would. This means that on standard resolutions you would have the exact same performance from an old 8800GT card at you would from a 980ti.
You would be more than likely better to stay at 2x Native on your system. Still great Gfx, but much more stable. If you push it up to 4x (1080), then you are going to start having either huge lags or skipped frames if using hacks on games such as Shadow of Colossus etc.