Anyone who cursorily looks at public transport logistics realizes that time deficiencies almost never lie on the actual motorization method. Electric, diesel, rail, rubber, car, bus, train, etc. All of those factor's influence pale in comparison to embarking and disembarking times.

It doesn't matter if you can make the trip in 15 minutes or an hour, if you always have to wait 40 minutes to disembark, then that trip is always capped at 40 at the least time it can take. The Vegas tube terminals are absurdly small. Thus people have to wait a long time to board a car, which isn't the most efficient thing to get on or off. And they have to wait a lot in line before getting to a park spot to disembark. Then it's the fact that each has to be driven by a person who need regular food and bathroom breaks and general rest. And there's a driver per every 3 or 4 passengers. Inefficiencies begin to build up.

So, under one metric, from departure to arrival, yes the tube itself could carry 4k people an hour. But as a transport system as a whole it is awful at capacity and collapses as soon as so many people actually try to use it. This is a system that experienced a traffic jam inside the tube in their inauguration day, because that's just what cars do when so many are at close proximity.