I've been to a good number of book conferences these past few years. I've observed the following about possible speaking time slots (with respect to the attendees):
up to 9 a.m.—too early; caffeine hasn’t kicked in for some; people straggle in throughout
11 a.m.—mind starts to wander to food
12-1:30 p.m.—competing with lunch is like having a dance-off with Shakira
2 p.m.—digestion lethargy; they’re not nodding in agreement—just nodding off
3 p.m.—see 2 p.m.
4 p.m.—feels like the end of the day even if it isn’t; a time when many slip out
5 p.m.-7 p.m.—dinner
That means the most desirable slot is 10 a.m. Too late to be groggy, too early to be hungry or leaving.