I'm skeptical of r/w speed affecting the seed at all, and I know this is becoming a trend of mine, but I'm currently not in a position to test this theory.
What I'd like to see is someone with say, 100% faster r/w speed run a rack until they're consistently getting one item, and then take all their faster r/w charms and equipment off and run it again. If the rack consistently drops a different item, then I'll begin to believe it.
At this point, I'm still leaning towards writing it off as an issue of accuracy in where you're clicking the rack--I've found the seed can be very finicky at times.
I should note that if your r/w speed does affect the seed, then there might be some interesting uses for that knowledge. Off the top of my head I'm thinking a paladin with maxed vigor could manipulate the seed much like amazons, necromancers, druids, and assassins can.