100% of utilization increases lead time

Excerpt from the book Accelerate [1]:

Queue theory in math tell us that as utilization approaches 100%, lead times approach infinity - in other words, once you get to very high levels of utilization, it takes teams exponentially longer to get anything done.

Obs: lead time is a measure of how fast work can be completed.

One clue why this happens: when “resources” (workers) are 100% busy and committed to given deadlines, it is impossible to handle (unexpected) incidents and keep the deadlines. And (unexpected) incidents happen.

[1] Nicole Forsgren, Jez Humble, and Gene Kim. 2018. Measuring Performance. In Accelerate: The Science of Lean Software and DevOps: Building and Scaling High Performing Technology Organizations. IT Revolution Press.