There was always something odd about red dot scopes in ArmA series, and now i finally detected what it is. The scope crosshairs are motion blurred. It makes absolutely no sense as the eye position remains same in relation to the scope crosshair, yet when you start to look around, the crosshair is motion-blurred by the same amount of blur as the far terrain you are looking at.
Not only it looks weird and sends signal to your brain something uncanny is going on, but it also makes red dot scopes feel somewhat hazy, laggy and unnatural when in motion.
So that is why ironsights feel a lot more steady and agile. It is because they do not suffer from non-realistic motion blurring.