Mouse "randomly" slowing down or stuck in certain places (especially while starting a drag)
I just found this because I had the same problem. I noticed that the cursor would come to a crawl whenever I hovered across a list of links on a website. The culprit: I had my mouse cursor set to a larger size. 

Solution: Go to System Preferences › Accessibility › Display, and set the cursor size to Normal (all the way to the left). After restarting Safari, the problem was gone. I can move across a list of links, the cursor switches between arrow and hand, without lag.