Depends on what's holding it back.
Humans, like all engines, can only turn the pedals with a certain amount of force and at a certain speed.
If you feel you have power left, but can't spin the pedals any faster, then a bigger sprocket(by the pedals), or a smaller driver (at the rear wheel) will give you more top speed.
But you'll need to push harder to get the bike going.
If you have spin left, but not more power, then going to a smaller sprocket or a bigger driver might give you more top speed. It'll make it easier to get the bike going too.
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We used to convert our old Schwinn Stingray's to BMX bikes, and they were extremely heavy, but bulletproof. The bikes made today aren't built as sturdy as the old Schwinns were but that doesn't mean you can't convert a bike and have fun with it.
Modern BMX's are built from first assembly to be rugged and strong, that can't be added on later.