If you look at an airport diagram, (they can easily be found online), you'll see some taxiways that are at a 90 degree angle to the runway. An aircraft would have trouble making a turn onto one of those unless it was moving slowly. Other taxiways are at an angle to the runway--aircraft don't have to slow down nearly as much to exit on those taxiways, thus "high speed".
The less time each aircraft spends on the runway, the more traffic an airport can handle.