The only reason that I can think of is it might be that its a fun fly and I don't think they are designed to fly fast. I guess you could try really slow flight then go directly up and apply full throttle because then the power is the same but the speed is slower. If the tail wags then it's probally a engine thing and if it does not its probally a airframe aerodynamic thing. (I'm using "thing" in its scientific term by the way )

Hope that helps,
Alan