True, but then you spend all day in a hotel/airport/crash pad.

A high speed is a line of flying where you usually operate a late flight out of your hub, have a four hour or so stay at a hotel, then fly an early flight back and are finished for the day. Also called CDOs or Continuous Duty Operations, as you are on duty from the start of the first flight to the end of the second, complying with 117. Some folks like them, some hate them.

For example
9pm-10pm DTW-CAK
10-4am Hotel
5am-6am CAK-DTW

You are on duty from 9pm-6am, in accordance with 117, hence the short stay at the hotel, rather than the 10 hours required rest.