When you project out how long it's going to take you to complete something, and build a calendar and actions around that -- what pace do you choose?
I've found most people who are driven (so, not intentionally slacking or setting things artificially easy) are going to pick one of two paces:
Either (1) the realistic pace, which accounts for delays, bumps in the road, etc, or
(2) the max sustainable pace, which is what you could theoretically do if you didn't get off track.
I'm pretty well-convinced now that #2 is a dangerous way to plan.
The fact is, thing do go wrong. Which would be fine if you could recalibrate without it effecting you, but most people get neurotic and demoralized when they're missing their targets and deadlines and falling behind a pace they've set.
There is at least one major downside to setting a realistic pace, which is Parkinson's Law: "Work expands so as to fill the time available for its completion." So, you might unintentionally/subconciously go slower than you could if you've set a pace that has more slack.
Despite that, I think the potential Parkinson's Law disadvantages are greatly outweighed by the demoralization/neurosis of setting a max sustainable pace (and inevitably, eventually, falling short).
Humans are fallible, we're not robots. If you set the highest realistically possible pace, eventually an illness, accident, or whatever will throw you off, and then things can cascade pretty quickly into problems. While it seems less heroic and triumphant on the surface, building slack into your projections and giving yourself room to breathe is probably more conducive to achieving things consistently without going crazy.