Correction: It was not an optical illusion.

In fact the DeltaTime (the time between two rendering steps) is 7 ms in window mode and 17 ms in fullscreen mode. I have no idea why, but that's not the problem, as it still looks fluent. The real problem about it is that NewtonUpdate seems to be slower. I mean the objects fall slower.

Code:
NewtonUpdate(FNewtonWorld, DeltaTime);
DeltaTime is the time since the last call of NewtonUpdate. Is there any possible reason for this strange behaviour?