[https://forum.unity.com/threads/cinemachine-freelook-x-new-input-system-crazy-sensitivity-issue.927714/#post-7423121]
Reproduction Steps:
- Setup a FreeLook with Mouse Input
- CM Brain must be set to update on FixedUpdate
- camera rotation speed affected by FPS (test this by limiting the FPS with Application.targetFrameRate)

Reason for bug:

Mouse input is inherently frame rate independent. Unity’s input is just a wrapper for direct x input. Direct x queries the input state every frame and makes the data it has aggregated available to us (Unity). This is why you need to read input in the normal Update function and not FixedUpdate. It needs to be read at the same frequency that direct x makes the information available. Unity gives us the distance the mouse moved since the last time direct x made a query.

Unfortunately CM vcams read and process input whenever they are updated by the Brain - and that might be FixedUpdate.

The fix would be to add a mechanism that ensures that input is only ever processed once per render frame.