ummm, any time you go to a smaller rear sprocket, you will lose low end torque and take off power.
Look at a 10 speed bike, assume that you're peddling it....1st gear is a big sprocket on the hub...VERY easy to pedal from a stop, lots of low end torque, you won't get much speed because it's so easy to pedal, you will get up to the top speed quickly. it takes very little energy to get the bike moving, and it takes a lot of energy to keep the bike going at a decent speed.
5th gear is a tiny sprocket on the hub, hard to pedal from a stop, no low end torque, but once you get it going, the peddling is easeir and your speed is pretty fast. it takes more energy to get the bike moving, but once it's moving it takes very little energy to keep it going at a decent speed.
you will ALWAYS make a trade off when you swap rear sprockets. bigger than 44 tooth will give you more low end torque, less top end speed. smaller than 44 tooth will give you less low end torque, more top speed...you can't have both unless you go with a shift kit.
what size wheels and tires are on your bike? taller wheels and tires will giove you more speed, shorter wheels & tires will give you more low end torque.
the wheel and tire size also limits the top speed / low end torque.
how much do you weigh?
rider weight can also limit top speed and low end torque. if the bike has to pull 150 pounds from a dead stop, it will get up to speed faster than if it has to pull 220 pounds from a dead stop. but, weight shouldn't have much to do with the top speed because as we all know, heavier objects can move faster than lighter ojects once they're moving. (compair a car to a freight train.) a car and a train can both go 100 mph, but the train has a lot more weight behind it...more inertia, which takes A LOT longer to get it stopped. the car can hit 100 mph quicker than the train because it has less weight to pull.
but if you are not getting any more speed out of it with the smaller rear sprocket, it is possible that your carb is not tuned right, to allow the engine to rev like it should.
if it is too lean, the engine will bog at w.o.t. and the speed will be limited by that.
as the aleman said, no 2 engines are alike....there are screamers and there are lemons.