thehen - what resolution is the game running at?
We've seen issues on the iPad 3 where a 2048 size texture is very slow, but a 2046 size texture is very fast. My theory is that it internally adds a one pixel border to whatever you ask for, pushing 2048 over the maximum supported texture size. Then the GPU is actually rendering to two 2048x2048 size image tiles, which halves performance.
If you're running at 1366x768, it's possible the GPU is working with a 2048x2048 texture, or two 1024x1024 size textures. 75% of that, rounded down, is 1024 - so it can use a smaller render texture, or just one where it would have used two. I'm not sure if this qualifies as a bug, but you might want to make someone at MS aware of it. To test the theory, try 74% (which is just over 1024), and it should be slow again.