This is a little simulation that I have run numerous times. It shows in a very direct and apparent way that the less things we do at the same time, the faster each thing runs. In other words: Limit you WIP!
I like this exercise since it can be run by a single person or by many (64 is my record) and still get the message across.
This presentation is a simulation of the simulation, so if you click through the slides fast you get a little animation feature as well as timing.