From the Super Mario Wiki
A Speed Block is an item that appears in Mario Party. Other than the green color, its appearance is similar to a regular Dice Block. It can be bought at the Mushroom Shop for 200 coins. When a Speed Block is activated, it can spontaneously appear in place of the Dice Block on a player's turn. When it appears, the player can only move 8, 9, or 10 spaces, guaranteeing that the player will move a great distance. A Slow Block is the total opposite of a Speed Block.