The Spread function is lagging on my laptop and I have, for a long time, tried to find a way to disable it.
Now I have to search no more!
Unity Tweak Tool to the rescue.
Open the program, then goto
Window management -> Window spread -> Disable.
Spread is used when you click the app icon in the launcher and there are (at least) two available windows for that app.
Installation instructions on
Lifehacker.