They are not fast "right out of the box" - no buggy are. The engine has to be breaked in properly before you can tune it for speed. After the tuning has been done, it's fast. And remember that on a track it has more to do with the drivers ability than the outright speed. BTW it is easy to change the gearing according to your wishes regarding topspeed and acceleration.
I haven't tried the engine that normally comes with the Comp, but there is a guy at my local track who has a similar engine and it is amazingly fast for a standard rtr-engine and it was not that difficult to tune.