The G4 is a good chip, but it has it's problems. It has a slow Front Side Bus and doesn't clock very high, but it is pretty efficient. The Centrino (which is just a fancy term for a Pentium M [Mobile] and wireless 802.11x) is based on the old P3, which was much more efficient than the P4, but didn't clock as high.
The problem is, you were looking at a low-end machine. The iBook is great for what it does, but it's not that fast. Especially with it's 4200RPM drive, low-end video card, and (I'm assuming) low amount of RAM. Coupled with the fact that this was probably a machine that had been used quite a bit, there's your slow down. How many other programs were running? Had it been rebooted or re-imaged recently (probably not)? I had the same feeling when I played with an iMac at an Apple Store, but when my friend got one, it was surprisingly quick. Especially when we upped the RAM to 1GB. Made me think that maybe I should get one too.
One other thing you might not realize. When Windows launches an application, it pops up right away most of the time. But for some things, you can't actually use the app for a few seconds until it is finished loading. With a Mac, it takes a few seconds to load some things, but when it pops up, it's usually ready to go. With a good video card, you can see how smoothly things like Expose and Dashboard runs. And with lots of RAM, almost everything basic runs instantaneously. Even when you runs several things at once. Windows on a fast CPU will seem a bit snappier for some things, but an app taking 10 seconds to load is
NOT normal for OS X, even on an iBook. Even Photoshop or FCP. MS Word maybe.
