At Join Avenue's Market, X Speed can be bought in dozens at $3150 each, which equates to $262.5 per X Speed.
Effect
Generations I to VI
When used from the Bag during battle, it raises the Speed of a Pokémon currently in battle by 1 level. From Generation II onward, it also increases the Pokémon's friendship a little.
Generation VII onward
When used from the Bag during battle, it raises the Speed of a Pokémon currently in battle by 2 levels. It also increases the Pokémon's friendship a little.
Wonder Launcher
The X Speed appears as four Wonder Launcher items, increasing the Speed stat by 1, 2, 3 or 6 levels. It costs 3, 5, 7, or 11 points to use respectively.
X Speed (along with X Attack and X Defense) appears in Super Smash Bros. for Nintendo 3DS/Wii U as equipment that any of the five playable Pokémon can use. X Speed raises the holder's Speed while lowering Attack. While it will occasionally come with a special effect, the ones without effects come in three rarities: normal, Threefold, and Sixfold.
After rescuing a distressed Mii in an area, they will occasionally use X Speeds (along with X Attacks, X Defenses, or Potions). What they give out is chosen randomly upon being rescued. The X Speed boosts the Speed stat for some time.
X Speed in Pokémon UNITE
Pokémon UNITE
In Pokémon UNITE, the X Speed is a battle item unlocked at Trainer Level 8. It raises the player's Pokémon's movement speed by 45% and makes it immune to move speed reduction for 7s. It has 50s cooldown.