I've never really been one to pay much attention to my dogs speed on course. Basically all that concerned me was getting out there and having fun, running fast seems to be fun so we try to do that. However, course time would be good to calculate every once in awhile to know how my dog is running compared to how she ran previously.
Here is a link, to a chart for calculating dog speed on the agility course. You just punch in the course yardage and your course time and it calculates the dog's speed in YPS (yards per second) and MPH (miles per hour).
Check it out.