The motor has to put out more unneccessary effort to move a bigger wheel. More effort is also required to move a heavier wheel. Being a bigger wheel means the gear ratios are off, thus giving you the slower pick up. The extra weight makes the car struggle more at the moment of inertia and use more power to keep the drive wheels turning. So basically, you're straining your motor to look good. Of course, a modded engine helps, not as much as if you removed those 18's in favor of 15's or 16's.
