It's the same reason you can probably sprint 1/10th of a mile in 30 seconds, but can't run a full mile in 5 minutes. Doing anything for a sustained period of time is a lot harder than doing it short term.
BTW, you need to work on your math. 1 hand per 13.5 seconds would only be about 267 hands per hour.
You will get 500 hands per hour done if you play a hand every 7.2 seconds.
Can you really play a hand every 3.6 seconds at a sustained rate for several hours? I have my doubts, but I know that the vast majority can't.