I'm not sure about the speed but because the bike is a 250 the majority of the power is made at high Rpms as all bikes, just the larger the motor the more power made at lower rpms. The bike if it has had exhaust or other mods may need the carb re-jetted to compensate for the changes if not already done so. However, I would assume it's just the gearing of the bike. It's imperative to lunricate the chain as well especially on smaller engines as the sprung weight and friction are what hinder the bike the most. Hope
This helps
This helps