A light flywheel helps acceleration at all speeds your car can attain, it just speeds up the acceleration. Imagine you have a stick with a heavy weight on the end and you have to rotate it as fast as you can. It will be hard to do and you will be slow at it right? Now remove the heavy weight (say it's an 18 lb weight) and replace it with a lighter one (a 7 lb weight). Rotating the stick with the lighter weight will be easier now and you will be able to accelerate your spin faster now right? Same concept for your light flywheel...