If the wheels were identical in every way, including in levels of hub resistance, except that the new rim was substantially lighter than the old rim, the difference in rim weights would explain your results. Less mass means less inertia.

Example: drop a ping pong ball into water. Then drop a steel ball the same size into water. The lower mass of the ping pong ball accounts for the fact that the resistance of the water halts the ping pong ball almost instantly whereas the steel ball is barely slowed.

If the new wheel has a lighter rim, there's less inertia, and so wind resistance becomes a more important factor.