unity slow with dual-monitor

Solution 1:

If your laptop has an Intel N10 graphics chip, it may be this bug: https://bugs.launchpad.net/ubuntu/+source/compiz/+bug/830949.

If so, an easy way to test is to lower the resolution of the external display until the sum of horizontal resolutions totals less than 2048.

If it turns out to be that problem, you have several options. One is to arrange the screens vertically rather than horizontally (the idea is that everything must fit in a 2048x2048 matrix, so as long as your monitors don't add up to more than 2048 in either direction you should be fine.

The second option is to use Unity2d, which doesn't trigger this bug.