Mouse "randomly" slowing down or stuck in certain places (especially while starting a drag) Since upgrading to Sierra (but could be other cause), my mouse slows down in certain points (as if something is intercepting the mouse move event and swallowing it). Sometimes this is accompanied by mouse cursor blinking between rapidly between different cursor shape, in a certain point which allows drag&amp;drop (only/especially when drag is started); then the mouse pointer becomes "stuck" as if moving in dense syrup, for a few seconds/pixels. But this happens as well when not drag&amp;dropping as well. 
 Though, the effect is weaker (though still negatively affects productivity).

An extreme case is when doing native HTML5 drag in Chrome...

Please note: this is not about scrolling (unlike some questions which seem similar).

MacBook Pro Retina 13", late 2013. Two external DisplayPort monitors.

If this could be blamed on an app (as opposed to macOS sierra itself), then which kinds of apps would have such power to intercept and affect mouse-move events?
Personally in my multiple decades of computing, I've never seen mouse-move being affected in such a way. Usually the mouse was moving the way it should...

Tried Safe Mode (holding shift while booting), but it completely fails to load (progress bar stuck at 100%)...

Time for a new macbook? Or time to go to Windows? :)

This is ruining my productivity (mouse moving at random speeds, sometimes 2-10 times slower).

Thanks for any hints!
Does affect magic mouse 2 and logitech usb-cable mouse.

Problem does not seem to happen on MacBook's touchpad.
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.