I don't work yearlings in small circles, not in a round pen nor on the line. Ours get worked free in a large corral, 60'x120'. I just don't like all the circle work on young legs.
But, if they must be worked in the round pen I wouldn't go so much for a fast trot--just a good working trot where the horse is using himself. That is the best way to tone them up & ut some muscling on the hindquarters. I'm not sure, of course, just what you call a fast trot but I know some people that have their horses just scurrying around the the circle--the horse is trotting fast but not using himself at all--and then they wonder why they don't see the horse getting into the condition they were hoping for.