A great article on why
The solution given in the article? two things: 1) Get Apple to optimize OSX (which Apple is doing and will just take time). 2) get more memory! MacOSX is very memory intensive, but is speedy when it has memory to gobble.
Which leads me to my point. Why the HELL doesn't Apple just sell 256 megs of RAM with every single machine they sell now. The system requirements for OSX is AT LEAST 128 Megs of RAM, and yet that is the base for most computers (and some actually sell with 64 megs of RAM). It is like the original iMac that required at least 40 megs of RAM to have OS8.x up *and* Netscape.... immediatelys sending anyone with only 32 megs of RAM into virual memory and feeling slow. Given opening Netscape was the primaty purpose of the machine to 99% of the people who bought it, this was unacceptable.
RAM is cheap. RAM is super cheap, and it is the most cost-effective way to make a machine feel zippy even when its processor is a little old *cough g3 cough*. Apple should base everything on having 256 megs of RAM and be done with it.