If I have two monitors, side by side, then the Unity bar will interfere with mouse movement. I set the unity bar to only display if I move the mouse to the top left corner of the screen, but the mouse still pauses momentarily when I move the mouse from the right display to the left display. The place it pauses, is exactly on the left border of the right screen.
You must have sticky borders enabled. Remove the option sticky borders.
A similar problem has been answered here: How do I disable mouse magnet on middle edge with multi monitors?
Another post here will help, and provide you with some options to set things up the way you want.