That is awful. Really disappointing. How come they still can't build a snappy interface? The Nexus 7 destroys this. What a shame, I had high hopes but now it looks like the new Fires are going to be hard to recommend...
My guess is that they ripped out a significant amount of the UI code. The original Fire was based on Gingerbread which had no hardware rendering. ICS came along but they continued to build on their own front end code (it wouldn't be worth throwing it all away and starting again) so it still lacks hardware acceleration.
They could also not be threading and instead just blocking the UI thread.