the LIRR also goes from manhattan to jamaica, and it does in fact take 20 minutes or so (21 according to google maps)

You can only pick up the LIRR from midtown and the Jamaica endpoint is the AirTrain which requires a separate fare and another 10-12 minutes to get to the terminal. Having said that, AirTrain/LIRR is my preferred route into Manhattan on business travel, but I always take an Uber out to the airport unless I've got a late night flight.