Just speaking for me, if I'm riding at 20 mph, I'm focused solely on riding; my head is probably down, I'm in much more of an aero position, and I'm constantly scanning the road ahead for potential obstacles, etc. because I want to have as much time as possible to react to a changing situation. I'm definitely not spending a lot of time enjoying the scenery. At 10 mph I have the time to watch deer in the field, see the coyote running through the bushes, watch the hawk soaring overhead, etc. As I said, this is just me; if your level of conditioning and reflexes allow you to watch the scenery and wildlife as you pass by at 20 mph, more power to you.