I think you're already aware of this, but I'll post it again and maybe later on record a video.
I have my fps capped to 60fps, however when I ads I lose frame and it drops to 50+ then immediately back up to 60 so long as I remain ADS.
In a first person shooter this is critical, maybe even gamebreaking for firefights.
I find that if I have my fps capped to 120fps and then ads I lose even more frames!! Around 105+fps then it will go back to 120fps.
This happens with all optical attachments. I believe with the 2x scope and above the frames you lose is more than 1x optics.