Acela's average speed is 82 mph between NYC and Boston. 66 mph between NYC and DC. When you add the time to get to/from the stations my point makes a lot of sense.
If you remember the history, many of the existing rail lines were built before the suburban explosion post WWII. To make Acela go faster, the route would need straighter track which would require buying a lot of expensive, already developed, land.
That’s the basic assumption for any HSR plan. You can’t mingle 160+MPH passenger trains with freight, so you need to buy a lot of land anyway. However, getting out of the urban center at 80+MPH isn’t a major issue and that let’s you avoid buying the most expensive inner city land. Getting off in the center of a city like Penn Station vs Laguardia Airport more than makes up for a few miles of slower speeds especially as these trains take a while to accelerate.
If you remember the history, many of the existing rail lines were built before the suburban explosion post WWII. To make Acela go faster, the route would need straighter track which would require buying a lot of expensive, already developed, land.