Im listeting right now to some of his sound samples and i can tell that Art Tatum played much faster.
nooooo sorry. Lubomyr Melnyk is the fastest pianist ever recorded. He makes up for it with his really crappy music though ^^ One of the worst composers I've ever heard.....
Any piece can be the fastest piece as long as it says "presto posible" on it. Which literally means "as fast as possible" so this is kind of dumb. Also anything with "trill" in it could be the fastest piece ever written depending on how fast you trill. I think Finnissy's all.fall.down. is rediculously fast, as is Xenakis's Herma, which requires at some points more than 20 notes per second. Alkan's Comme le Vent is not meant to be played as fast as that midi; look at the sheets =P
Pianists such as Mei-Ting Sun (not trying to start a fight about MT here) will take pieces like the Chopin Etudes and random Liszt compositions, most famously Feux Follet, and play them rediculously fast, probably beyond the speed of any of the aformentioned pieces. The Prokofiev Toccata has also been recorded at incredible speeds; as has the Schumann Toccata Op. 7.
But in actuality, the fastest piece outside of theoretical music probably was written by Lubomyr, and the fastest playable (if you want to call it that) piece is probably Xenakis's Herma.