Some people call this “phenomenon” planning against the deadline.
A friend of mine was a postal worker for a brief period a couple decades ago, and at that time (in his location at least) he was told to make sure he took the full day to complete his route, because if there was a consistent pattern of finishing early they would lay off some of the workers.
It's not always possible to make sure incentives are well balanced, but managers should at least take a minute to put themselves in the shoes of the employees. If finishing early has only negative consequences, then maybe that could be addressed directly instead of just setting impossibly early deadlines
If I have 10 hours to make a car trip, but it only takes 5, I don’t drive at half the speed limit. Sure, there may be some extra bathroom and snack breaks, but probably not 5 hours worth.
I see people underperforming due to low expectations quite often, possibly following the classic 80/20 rule. I think this “planning against the deadline” behavior is one of the difference-makers between a hungry startup that outperforms, and the legacy player that barely keeps up despite having 10x the staff.