lost horse power

Head gasket leak.
Debris in the carburetor.
Fuel valve clogged.
Gas cap not venting.
Air leak at the carburetor to manifold or manifold to cylinder.
Severe air leak at cylinder base gasket.
Air leak at one of the crank shaft seals.
Wheel hub(s) need servicing/ coaster brake dragging.

Do you and/or the bike weigh more? 5 extra pounds can cost you maybe 1 mph. ChinaGirls don't produce but 2hp stock, so any extra weight is noticable.

Bad Karma, no feng shui or you ate too much pizza and drank too much beer!

On the serious side, agree with Gearanut or it could be a fouled plug if your mixture is too rich or could be simple as a loss of tire pressure, creating more resistance. Worn rings. Wind, humidity and atmospheric pressure can also affect performance. A dragging brake pad or disc can slow you down. A little bit of this and a little bit of that can add up to make your top end speed diminish.

Good Luck,

Chris
AKA: BigBlue

Do a compression test, But this is unlikely to tell you much unless you had a previous compression test to compare to.
Quite likely crankcase seals, or blocked fuel supply in carb jet.
I would start by over hauling carb and checking Mag side crankcase seal.