I had a similar problem with Minecraft - that is, until i upgraded to a gaming computer, try doing that, yes, i know they are expensive, but its worth it, and if this doesn't fix your problem, i don't know what will, also, i have had US2 only whilst on this computer, and i don't get much FPS lag, only really lag when the collisions happen and accuracy thing
The reason computers are expensive is because they are good, if you got a rather cheap computer, its probably pretty bad, no offence