could be a fuel restriction, dirty air filter, poor gearing, chain may be too tight, obstruction in exhaust, clogged exhaust, dirty head, too large of a head gasket, you may just have slow bikes too, try porting out your intake, opening up the airbox, just be careful how lean you go, you may want to upjet your carb if you are going to open up the airbox so you don't seize up your engine. there are tons of reasons bikes go slow. there are just as many solutions to make them go faster!
if i can be of any help shoot me an e-mail
swarm and destroy