Remember, though, that the speed that maximizes line capacity is not necessarily the
highest civil speed. Higher average speeds mean that you need longer signal-system enforced "wakes" out in front of trains because they take a longer distance to stop from those speeds. Longer wakes = longer headways = less trains.
So, ironically enough, 60 MPH enhances capacity on the High Line by shortening headways. No fun if you're used to sailing through at 90 MPH, but it's certainly not a tragedy for capacity. While higher speed certainly makes the trip more attractive, right now that's overshadowed by the need to get as many trains into PSNY as possible, so capacity wins out over reduced trip times. The tension is between pushing as many seats across the High Line as safely possible in a given time period versus pushing a fewer number of seats at a slightly higher average speed, and at this point in history the number of seats is the priority.
Also - remember that all of this new traffic is not going to PSNY; much of it will be going to the new station at 34th Street (so there's, what...maybe two out of the five waterfront terminals?).
The High Line, under optimal conditions, can support up to 25 TPH - but it's a house of cards; one thing goes wrong and the pooch has been screwed. So 23 TPH is about the limit that Amtrak and NJT are willing to schedule through the North River Tunnels right now.