The relevant time is from whatever the transport hub is to the destination. The people who live a few stops from Heathrow on the Piccadilly line can get there pretty quickly but that is not a useful measure. I chose Penn Station because that's where the LIRR currently leaves from. The Wikipedia article talks about how a direct train line was planned but didn't eventuate.
I can't agree that NYC has well designed public transportation. Metro North, the other train line I have experience with, is also dreadfully slow. The subway is old, dirty, and has poor signage.
> The relevant time is from whatever the transport hub is to the destination
It seems like the Heathrow Express comparison is fair then; Paddington's a transport hub (surface trains and several tube lines pass through it), and it takes 15 mins to Heathrow.
I can't agree that NYC has well designed public transportation. Metro North, the other train line I have experience with, is also dreadfully slow. The subway is old, dirty, and has poor signage.