I prefer the term robustness to slack. And the concept applies to more than just time. Keeping sufficient inventory to deal with spikes in demand, having redund

I prefer the term robustness to slack. And the concept applies to more than just time. Keeping sufficient inventory to deal with spikes in demand, having redundant systems so you can keep running while doing maintenance, programs designed to fail safe rather than leading to cascade failures, these are all ways that systems can deal with the inevitable perturbations of the real world. The reckless pursuit of efficiency leaves systems fragile, which may be fine in good times but is catastrophic in bad times. In the long run, you need robustness.