From the Super Mario Wiki
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A Speed Flower is a type of flower found in Super Paper Mario. If one of the four heroes (Mario, Luigi, Bowser, Princess Peach) pick it up, everything will speed up for a short period of time. Within that time, points and coins the heroes would earn will be tripled. Mario's Flip Meter will also drain and refill faster. If an item that increases the player's points is used the points will also be tripled.
The Slow Flower is the blue-colored opposite of the Speed Flower, and makes everything go slower but triples the points and coins earned as well. If a hero grabs a Slow Flower while under the effects of a Speed Flower, the game reverts to normal.