Depends a little on the type of work (contract work is different), but generally disagree. Just because you can do the work in less time doesn’t mean everyone can. You can use your spare time to do other things, but if you’re only showing up 20h/week for a full time job, there’s no reason to pay you full time wages.

There’s something off kilter with the labor system when some people are doing the work of several workers while others are doing the work of half.