Speed Booster

The Speed Booster is a power-up that first appeared in Super Metroid in Norfair right after Samus defeats Kraid and obtains the Varia Suit. It also appeared in Metroid Fusion when Samus defeats the similarly fast Serris in Sector 4 and in Metroid: Zero Mission when Samus defeats Kraid in his lair. The Booster is set to appear in Metroid: Other M as well.

Super Metroid manual
"These boots allow Samus to run at a super high speed and make her invincible as she runs through enemies. Blocks with the Speed Booster icon melt away when Samus runs into or over them at super speed."

Metroid Fusion manual
"Dash at supersonic speeds and crash through certain barriers and enemies. Press and hold the Control Pad in the direction you want to dash."

Metroid Zero Mission manual
"With this power-up, Samus is able to dash at supersonic speeds and crash through certain barriers and enemies. Press and hold + in the direction you want to dash."

Zero Mission Samus Screen data
"Run until speed boost begins to break certain blocks and damage enemies."

Description
According to the Metroid: Zero Mission Instruction Manual, p. 26, and the Metroid Fusion Instruction Manual, p. 30, the Speed Booster allows Samus to run at "supersonic speeds." This indicates that she can move from her normal speed to Mach 1 to 4 within a second or so.

The Speed Booster, as the name suggests, will greatly increase Samus's speed whilst she runs. She also becomes invincible, as noted in the Super Metroid Instruction Manual, p. 24. In Super Metroid, the Speed Booster would only kick in if holding the dash button, but in Metroid Fusion and Metroid: Zero Mission, this changed, due to the Game Boy Advance not having the X and Y buttons. The Speed Booster is notable for being used in intricate, difficult puzzles for expansions, though it is actually required to advance further in the game, on occasion. The Speed Booster can be used in Samus' Morph Ball form, though the High Jump Boots are required for this. Also, the Speed Booster has been shown to be quite a powerful weapon, being able to plow through most enemies at full speed. It can also be used to destroy Speed Booster Blocks. These blocks cannot be broken with any other weapon, no matter how powerful.

Uses
The Speed Booster is very useful in the following instances:
 * Break Boost Blocks.
 * Run fast.
 * Crouching while Speed Boosting to Shinespark.
 * Run through many enemies.
 * Do a very far and high jump if Samus jumps while dashing.

Trivia

 * In Metroid Fusion and Metroid: Zero Mission, if Samus stops running in mid boost, the player can hear the slight sound of Samus' boots scraping against the ground to slow down.
 * Samus is able to use the Blue Suit glitch when fighting Draygon.
 * The Speed Booster's Shinespark is a reference to 1970's mech anime Getter Robo G in which it was the titular robot's most powerful attack, however like its Super Metroid counterpart usage quickly drained it of energy.
 * An interview with Retro Studios revealed that the Speed Booster was originally being considered for inclusion in Metroid Prime, but did not appear due to difficulties implementing it. Unused scan images show Samus performing what appears to be a Shinespark.