An object at rest, tends to stay at rest.
An object in motion tends to stay in motion.
Heavier flywheel is slower to accelerate than a light one.
So, the bike revs up quicker.
It also revs down quicker, so the bike should have more engine braking with a light flywheel.
And, if you think about it, with less flywheel weight, there is less gyroscopic effect, so the bike should be easier to turn.
Heavy flywheels are nice for a clock like idle, and holding an engine steady at any RPM (smooth highway riding). They mask a lot of poor tuning issues.
Having said that, no one has said the bikes were worse after putting in a light flywheel.
One way to find out