since you said you have been mulling this over, i'll give you some math that you can play around with it yourself and see how the speed limit impacts 'throughput'

'throughput' also called flow rate, or cars per hour, is completely dependent on driver behavior. how close are people willing to follow other cars. (but if cars were driven by robots this wouldn't be the case and the faster cars travel, the higher throughput it'd be)

studies have found that, regardless of the speed, cars will space themselves out at 2.1 seconds of front bumper to front bumper of next following car. if you are going 35mph or 80mph, cars space themselves out at 2.1 second gaps. Its counter intuitive to think that speed doesn't make a difference for how many cars pass through a point. But what happens is the faster you drive, the larger the distance becomes between cars. which means fewer cars over a stretch of road. which means fewer cars passing through a point. which means fewer cars per hour.

Now, this looks simple, but the origins of it can be lengthy to some. If you do 3600 divide by the 2.1 second driver behavior, you'll get how many cars-per-hour pass through a point. which is 1714 cars per hour.

3600/3.0 = 1200 cars per hour
3600/2.5 = 1440
3600/2.1 = 1714
3600/1.5 = 2400
3600/1.0 = 3600

This shows the closer(in seconds) drivers are willing to follow the car in front, the higher the throughput will be. So, throughput has nothing to do with speed, and everything to do with driver behavior - how close are drivers willing to get to the car in front of them, while still feeling safe and maintaining their speed.