Kingman's equation (the VUT equation) approximates the wait time at a workstation as the product of three factors: Variability, Utilization, and process Time. The utilization term is non-linear — as utilization approaches 100%, wait time and WIP blow up toward infinity.
It is the mathematical reason lean keeps slack capacity and attacks variability: you can buy shorter lead times either by reducing utilization or by reducing variability, and reducing variability is usually free.