Rainbow six

You are experiencing the maximum turn speed limit built into Rainbow Six Siege as explained here. Unfortunately the game developers built in what we consider to be a low turning speed cap and gave it a funky, non-linear aim/look mechanic that requires adjusting the way you play to compensate. If you are a wrist flick type player (high sensitivity player) it will definitely take some getting used to versus arm movement style players who are used to longer movements for turning.

My personal suggestion is to try both R6S sub profiles (Classic and Updated) using the in-game settings listed on the profile download page for each, and see which one is less problematic for your style. Once you see which one is smoother for you, start increasing the sensitivity to your liking until you hit the turn speed limit where it starts going slower the faster you move the mouse. Get a feel for this speed and learn to work within it. You can also try using curves as explained here so that you can keep slower aiming speeds but have the curve kick in when you need to spin 180 quick.