Regardless of any lag, any MicroProfiler reports or any sort of that matter, the base camera scripts in PlayerModule are tied to frame rate (RenderStepped) and cause jumpiness when frames drop. Calculation of the movement delta uses frame delta, which causes behavior like camera rotation overshooting and such.
For example, a jump from 144 FPS to 60 FPS may cause the camera to move nearly twice your mouse delta along a given direction. These jumps are sadly incredibly common given that I run various background tasks in the meantime (I keep Ableton or Bitwig open because I compose music in these programs).
Expected behavior
Similar to the New Studio Camera beta feature update, the camera in playtesting and on the client in general is decoupled from framerate, eliminating the jumpiness whatsoever. This would probably require some sort of public API that handles this separate thread.