Originally Posted by graycyn
It's certainly possible it wasn't a good Paperwhite on display at the Best Buy. Still, it appeared to be working. The dictionary speed was really slow, even on my older Sony 350, my dictionary pops up almost instantly, the wait on the Kindle was excruciating in comparison. It could also simply be user error, I am not used to the Kindle interface. I think I was using a long press like I do on my T-1, rather than a double tap like the 350 uses.
You (or I, inspired by your report) have discovered an interesting behavior (or 'bug' if you prefer): when you first open a book, you cannot immediately select text with tap-and-hold. That gesture will be ignored (as will a pinch gesture to adjust text size). I don't know if this is what you found 'excruciating' but thought it was interesting to note.
But once you get one gesture out of the way ('successful' or not), all the gestures work as intended. Looking up a word is just a matter of a tap and brief hold (perhaps a third of a second, so if you say 'one thou-sand' to count off a second, you can lift after saying 'one') on the word. Nothing will happen until you lift your finger off (Kindle is waiting to see if you want to extend the selection).
On the Sony you don't have to lift your finger before stuff happens, but you still have to tap and hold. So there is some delay there as well. If I were to guess, you were holding longer than needed and thus you were simply preventing Kindle from doing your bidding.