I know nothing about competing in dressage. However, I noticed that the start times for each performance is in a 10 minute interval. I can;t imagine that a test is more than 3-4 minutes long. That leaves quite a bit of downtime between performances. Uh, doesn’t the crowd get bored? 
the Grand Prix test is about five and a half minutes long (5.40 IIRC), Grand Prix Special is 7 minutes, freestyle between 5.30 -6.00. just enough time for everyone to criticise/gossip/express horror at the blind judges’ marks…etc.
Judges and scribes have to ensure test sheets are filled out correctly. And it ensures that every ride can start on time. Good system.
Plus about 1.5 minute to ride around the ring, 45 seconds from the bell-signal to enter at A, 10 seconds extra to reach X, and then back again leaving the ring.
And offcourse 3 seconds to smile at the judges 
Theo
GP tests usually average 6-7 minutes plus the in and out time. I believe every 4 or 5 rides in FEI event the center line must be marked by rolling That is usuall included in the time schedule.
Some judges can get quite picky for a riders ride time. The coutresy pass should start before the official ride time and the rider should be addressing the center line when their ride time ticks.
You also need to send runners to all 5 booths to gather tests and you do need to allow a little fudge factor for unforseen mishaps like an arena rail down. Plus judges need time to review their sheets and mark the co-efficients.
There is also the safety factor of getting horses to and from the warm up arena.
10 minutes seems like a lot, but it is often just enough time with a small amount of breath. If something comes amiss you need to allow time for conference.
most shows I work or go to schedule the FEI classes every 10 minutes. You can get away with every 9 minutes but you better have a real good ring steward and it better not be too far from the warm up ring.
I will confess to falling asleep during the Atlanta Olympic dressage and I think it had something to do with the length of the tests or the interval between them. Also, it was hot and we were tired from getting up before dawn and then going on a series of “death marches” from car to bus and bus to venue entrance. I did not fall asleep during the jumping where the horses come in at shorter intervals and there is little to no down time. While you can argue that dressage is less dynamic to watch, I truly think the difference had something to do with having one horse out there for a long period as opposed to several different horses during the same time period for the jumping.
A funny and somewhat related story having to do with monotony in dressage: We were sitting next to a couple with two very small children during the eventing dressage. Although they knew nothing about eventing (or any horse sport) this was apparently the only thing they could get tickets to that they felt would be reasonably compatible with small children. They asked a couple of competition-related questions and then asked if there was a requirement that the horses be brown. I’d never thought about it before that, but there are a lot of plain bay horses in eventing.
BTW, my dad did sleep through the entire afternoon of cross-country at the LA Olympics and I have the photo to prove it.
