Is pile shuffling considered slow play?
The tournament rules now clearly cover pile shuffling:


  Pile shuffling alone is not sufficiently random and may not be performed more than once any time a deck is being randomized.


The description from the update bulletin explains a bit more:


  Section 3.9 (Shuffling): This change has to do with "pile shuffling." Pile shuffling alone is not a sufficiently random form of shuffling. Update states that a player may only pile shuffle once each time a deck is randomized. The one pile shuffle is allowed in order to count the number of cards in the deck.


So, pile shuffle once, and you're okay, as long as you still actually shuffle your deck - it will not be considered slow play. Pile shuffling more than once would be against the rules, though it's not clear whether it'd be deemed slow play or just improper shuffling.