Jittery + High sensitive mouse

Ever since the update, my mouse gets extremely sensitive, mainly when looking up and down and while looking left/right. I can't find a solution because the only thing I can find is to set priority to below normal. I do this and it fixes it, but now my fps sucks. BUT, once I close and reopen MC it changes back to priority: normal. I don't have Razer Synapse installed and I'm at a loss at what to do.