Okay boys
You get two fat chicks on a roundabout (play park one), try spinning the roundabout.
Swop these chicks for skinny wee ones, is it easier to spin the roundabout??
Yes it is.
It's not just about weight, its about the weight of the components that the engine has to turn.
Lighter spins easier, therefore quicker acceleration, better braking and handling
The weight saving for light weight alloys and 2 piece brake discs is about 10 Kgs a side, 20 Kgs an axle, not a small difference. Couple this with a race fly wheel and you will feel the difference