My experience is pretty much limited to GSMNP, but I would agree... the JMT has lots of switch backs that for the most part keeps the trail from getting as steep as I'm used to experiencing.
That's on of the complaints I've been hearing over the years in the Yahoo and Facebook JMT groups. Many people are stopping for the day around 3pm at the popular campsites.
And that's where we get back into different abilities by different individuals. I pretty much woke with the sun, rested an hour for lunch, and hiked until 5-6pm just about every day. But there was only 1 day that I managed to get anywhere close to 20mpd... and that was the day after I stopped early at 2pm after climbing the Golden Staircase.
LOL