I just installed Parallels, the OEM version that came with my MacBook Pro, and set up Windows 10. I haven't used Windows yet, but in OS X my mouse sensitivity is far too high. Unless I choose the lowest sensitivity setting in which case it's far too low. I searched google for this problem, and found lots of threads complaining about games, but I'm not playing a game. Just using regular applications.
Hello @MichaelM24, Could you let us know the following information for us to understand the issue better, 1. Is that an external mouse? Also let us know if it is wired or wireless. 2. Have you tried the mouse performance with Parallels virtual machines? If the issue is with the entire host Mac OS then you might have to check the compatibility of the mouse with your Mac.