I was watching this program this morning where dogs run, jump over hurdles, through tunnels, quick turns etc, chasing a fake rabbit on a wire, with the winner doing it in the least time with the least of missed or flubbed obstacles. It looked to me that the dogs in this category of the competition were all mixed breeds, with a heavy emphasis on terrier in their mixes. Most of them looked quite a bit like Jack Russel's, but not pure bred.
To my eye, these little guys looked just blindingly, lightning fast. That got me to thinking about banty-weight boxers and how fast they look.
That got me to wondering, I wonder if they're really faster than bigger dogs/boxers, or is it a trick of my eye that makes smaller appear faster?
If the dogs were mixes, it couldnt have been an AKC competition and your analysis of the sport is very close; a missed obstacle is a non-qualifying score. Its easier for the handler to keep up on the course with a small dog, which makes the dog look like its running so fast. A larger dog has to cut back on speed to better navigate the obstacles. A longer stride length will give a bigger dog an advantage in a sprint but the agility ring looks for controllability first and speed second which gives the advantage to the little ones.
Having said that, there are also breed differences to consider. A Jack Russell will easily out run many larger dogs, they are insane little monsters of the dog world. JRs are not for the faint of heart, even the mixes.
}:)>