There's quite a few reasons that I can think of:
* PPCs have a smaller screen than computers, and in order to make viewing easier everything gets scaled down (unless you change the settings) to fit the screen better. This is slow.
* The processor is slower compared to a PC anyway. If you try browsing the internet on an old PC (say around 600mhz, 64MB RAM like the Axim) you will find it noticeably slower. The processor is actually quite important as it is used to "render" the computer code (HTML) that makes up web pages into the readable formatted page you see whenever you look at a website.
* Pocket IE is a fairly slow browser.
Opera is much faster (but you have to buy it unfortunately, although there is a 30 day trial).
If you turn of images and use "Desktop" display mode in Pocket IE it should run faster but be more difficult to read. Also more basic sites with less images, less complicated layouts will be much much faster.