Can some one explain how my mouse is slower when ADS in game?

I am aware of that there are settings that control sensitivity for ADS and normal movement separately.

I am talking about rare occurrence where when I am ADS at enemy (even around them) my mouse movement is slowed down sometimes like the game has issues processing inputs and what is going on at the same time.

It happens sometimes only and only for some enemies. Usually it's fine. Meaning the mouse feels exactly like at fire range.

But sometimes when this "bug" hits I am unable to track enemy with the same precision. Like the sensitivity drops by 10-30% or something. The cursor feels like it drags.