Momentum of rear derailleur wheels? I have new derailleur and I am somewhat surprised that when I try to spin it with hand (no chain is mounted yet) it basically has no momentum, it almost immediately stops. Almost any other spinning part of the bicycle has much more momentum and spins for a while.

Granted, the wheels in derailleur are the lightest ones, but shouldn't they at least make a one full turn?

The screws are not too tightened because out of curiosity I loosened them already. No change.
The derailleur wheels are small and light weight, so have very small moment of angular inertia. The bearings they run on need to withstand a significant force due to the action of the chain, not just the weight of the wheel. So the bearings are fairly robust and lubricated. As a result there is a little "greasy sliding resistance friction" (for want of a better word) in the bearing. The resistance is small and its effect on riding resistance virtually imperceptible, but as the wheel's angular momentum is also very small, it spins down rapidly if spun by hand.

If the wheel was engineered like a fidgit spinner then it would spin for ages, but its bearings would fail within minutes.