These bikes only have 15 hp at the crank so it's worth checking for any rolling resistance occasionally. I check once a month, by lifting the wheels off the ground and spinning them up making sure that the brakes aren't binding and that the chain is giving as little resistance as possible, low tyre pressures also make a lot of difference on these bikes. If everything is ok then the problem can only be the engine. Engine wise i'd be looking at checking the oil level, coolant level, air filter and spark plug colour. It's worth mentioning that these little engines are tight from new and may struggle to pull in 6th gear but as they wear, they start to pull/open up, usually at around 16000 to 17000 miles mark. Iv'e found that a free flowing exhaust with little back pressure allows the bike to easily reach high speeds, but at the sacrifice of some acceleration.