What does phatspeed stand for?

"Phatspeed" is not an acronym and doesn't stand for anything. It's a slang term often used to describe something that is very fast or very speedy, especially in the context of computers, internet connections, or vehicles.

The word "phat" itself is slang for something big, impressive, or cool, and "speed" obviously refers to velocity.

So, "phatspeed" simply means something that is extremely fast and impressive.