less rotating mass, so the engine doesn't have to work as hard to spin it, but it loses momentum quicker/have to keep throttle on to keep it revving, and when you shift, if you take too long the revs die real quick.
but you'll climb through the gears quicker, how does a lighter flywheel affect engine breaking?

PS, basic questions like this have been asked alot, so if you have more you may be able to search for it and find the answer quicker