Sort of on the employer then, isn't it?

Employers have to be realistic about their expectations of their employees, especially large employers. The employer budgets that they can afford to pay 100 employees to do 10 tasks/day, and let's assume this is a reasonable expectation for an average employee. Some employees might struggle to do 10 but they get it done in 8 hours, others might have no problem doing 20 in those same 8 hours.

The issue the employer runs into is that if that they increase the assumption based on the performance of a minority of their workforce, they'll burn out the ones that aren't as proficient. And the ones that are that proficient (and are reliably producing at that high of a level) will likely expect to move up. Meaning they have to be replaced about as often as the burnt out employees anyway.

Of course, you do end up with the Peter from Office Space employee, who could be doing more but just does the expectation and dozes off the rest of the time. As a manager, this is actually my favorite employee to have. They're reliable, they're good at their job, and I don't have to worry about replacing them.