“Underperforming” implies that someone knows better- that someone knows that the work being done isn’t as good, or as much, or is smart as what could be done.
The problem with underperforming is that it quickly becomes over-expecting, and at that point, a person who thinks he is simply underperforming is actually just kidding himself. He actually is performing regularly, but is obviously capable of less.
A person can only “phone it in” for so long before “doing his best” becomes “phoning it in.”
I’ve met people like this, people who never got over the teen-aged phobia of caring too much. After years of “Phoning it in,” they are simply obviously capable of less.
Everything you have deserves nothing less than everything you’ve got.