I am currently in the process of converting a personal texture pack to 1.5 when I noticed massive performance drops after using the texture pack conversion tool by dinnerbone in comparison to 1.4.7 frames.
Upon investigation, after using default animations, taking out and slowly putting textures back in while looking at the Stitched textures PNG files I noticed that once the block stitched textures goes above 2048x2048 resolution to 4096x2048 the game goes from a solid 90+ frames per second to 2 or 3.
I have included both stitched images. The only difference between the two is that I have removed the mycel_top.png and as a result the image size has been increased from 2048x2048 to 4096x2048.
If needed, I can also include zipped versions of the texture pack, just wasn't sure of the best method to test this.