According to the theory of constraints (on which I am expert since I read 1 graphic novel, The Goal, and 1 novel, The Phoenix Project) systems without slack become exponentially slower until no work is capable of getting done anymore.
The reason for this are statistical perturbations in stochastic events. If a task takes a worker rand(1,4) units of time you can expect them to do day/1 to day/4 tasks in the day.
You look at the worker doing day/1 tasks and you say, "Well shit, that person is slacking most of the time. Most of these tasks take 2 units, 4 is very rare". So you ask them to work harder and impose rules so they must perform day/3 number of tasks per day.
You look at your constantly busy workers and you're happy. No more slack in the system.
But your assembly line grinds to a halt. Nothing ever gets done anymore. Everyone is busy all the time. Everyone's always working. But nothing is finishing.
Turns out any task that hits N=4 on the random curve, wreaks havoc and you fall behind. Then you have both yesterday's and today's tasks to do. You can't. The next day ... well the problem ends up growing exponentially.
Efficient systems have slack.