From the Super Mario Wiki
A Slow Flower is found in Super Paper Mario. If one of the four heroes (Mario, Luigi, Bowser or Princess Peach) use it, everything slows briefly. In addition to jumping and running slowly, Mario's flip meter drains and refills slower.
During the slowed time, points and coins the heroes earn are tripled, and if an item increasing the player's points (such as a Gold Medal) is used, the amount of points earned is tripled. Sounds are also affected, with the death sound effect from the original Paper Mario used when defeating a enemy, and the Super Mario Bros. theme can be heard in the background music when the Slow Flower is used at the beginning of Chapter 3-3, The Dotwood Tree. One of Count Bleck's moves has the same effect as the Slow Flower.
The Speed Flower's effects are the opposite of the Slow Flower; it speeds up the game, but it triples the points earned as well. The Speed Flower is red.