if your texturepack is large such as mesa's realistic TP it will take a long time to load but if your loading up simplecraft or something that you really have a problem or have a old or lower speced computer.
I find that the resolution and amount of animations really does increase the time it takes to load. Even more so if that pack changes a lot of the start menu. Because as you click the pack it loads the menu graphics and then you have to wait again if you select it.
The more detailed the texture back is and how big it is determines the time it takes to load. It also depends on how fast your computer operates inorder to copy everything over. I had the same problem on a medium-built PC but when I switched to my fast(er) laptop, it took less time.