Turn the three dials

The wait, as utilisation and service variability move independently. The clerk's average speed never changes.

Try this. Set the variability to zero and push utilisation up to 95%. Then put utilisation back to 85% and raise the variability instead. The second one hurts more.

The grey curve never moves: it is the same desk with textbook variability. Everything the blue curve does differently was bought or sold with variance, not with capacity.