Yes, the Toddler Flashcards move while you are dragging your finger, rather than after, so that's a good point about the difference. Occasionally, on TF my son will slide halfway to the next card, then change his mind and go back, and play around with sliding back and forth a bit -- I think he likes seeing the cards slide in reponse to his finger movement.
Increasing the slide speed to "very fast" just does seem to make your app feel a bit more responsive, though I haven't let my son at it yet. (I also tried turning off the slide animation, but prefer seeing the cards slide. Even though I've set the options so you can go to the next card by tapping, my son likes sliding.)
I've tried to make sure my images are not too big, but may play around with reducing them a bit further. I think the docs said try to keep them under 600x600, right? I just got a new 4th-gen iPod touch though, so will need to see how things look on that before deciding on what image size works best for us. (Unfortunately, the new iPod isn't usable yet due to battery issues which Apple is apparently working on.) The new iPod is higher resolution -- does your app display images using higher res on iPods/iPhones with retina display? (Personally, I'm not sure I really need it; just curious.) What is the actual resolution used? I could try to make my images exactly the right size. I generally display the cards in portrait mode, in full-screen mode.
A couple days ago, my son was swiping like mad and actually got it so that the wrong text was displayed on the picture (I don't remember what the sound was doing). The main lesson from all of this is, if you really want to stress-test your app, let a 15-month-old try it out.