[SmG Shane]

The drop in performance is roughly linear, and it is proportional to the number of pixels your hardware is driving. Thus, if you are not getting an adequate framerate at 2560×1440, then drop to 1920×1080. The number of pixels is reduced by about 44%, but the performance will improve by about 40%. Going from 1920×1080 to 1600×900 is a 30% drop in pixel count, but will give you about a 20% boost in framerate. Lowering the resolution will make your game look more pixelated, but that might be a necessary sacrifice in order to run the game at an acceptable framerate.