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