I'm wondering if Preordain is too slow in U/R aggo, too low impact. I feel like you want disruption open or a delver turn one. I think Jeskai/Xerox can get away with preordain because of a higher moxen count. Is this the case? How can you develop your board presence while still pruning your hand with Preodrdain? Is Preordain too slow?
I have been thinking a lot about this deck and how it is played. You = need the Preordains to fix your mana base. If you are playing the Wasteland version then you have 6 fetch lands, 4 Wastelands, 1 Strip mine and 6 mana producing lands and 1 Library.
Preordain helps you fix that draw to get what you need when you need it and it also allows you to flip your Delver.
If you cut the Delver and focus on playing Young Pyromancer and Snapcaster Mage then Preordain is a flashback target to get more cards if you do not have anything better. Then, there is the blue stew aspect of using cantrips to draw into delve spells.
I do not think that they slow you down. They get you Xerox, if you do not want that then you may want to get into a more disruptive Hatebear style deck.