The only difference between the thumbnail view and the full screen view is the size of the image. As you mentioned, the full screen image is larger and uses up more bandwidth. Do you see this when your iPhone is on Wi-Fi or 3G?
The full screen framerate is usually limited by bandwidth, even on Wi-Fi connections.
On local Wi-Fi, you should see a few frames per second.
On a remote Wi-Fi hot spot, you should see about a frame per second.
On 3G, Apple requires us to keep the framerate less than 1 frame every 2 seconds. If it's a poor 3G connection, it may be slower.
On EDGE, you'll probably see about one frame every 5 seconds or so, or as low as 1 frame every 10 seconds if you have a poor EDGE signal.
If the framerate on you're seeing is significantly lower than that, please let me know what framerate you're seeing and what connection type you're using.