It's also possible that this one day of intense work is unsustainable for 5 days, and this is just their working style. There's nothing wrong with that as long as this isn't done out of laziness.
So what if it's done out of laziness? There's no law that says that a lazy person can't participate in the economy without giving up their laziness. There are jobs out there that are well suited for lazy people. Maybe OP has found one of them.
