From the Super Mario Wiki
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.
The Slow Flower is similar to the Speed Flower however it makes everything go slower. The Slow Flower is blue.