I wouldn't say its faster than 98SE if installed on a PC with exactly the same specs. First of all Win 98 doesn't take up around 5GB of hard drive, its much less than that.
My other computer (bought in 1999) came with 98SE installed, and I remember it used to be 'very fast' (450MHz - top spec!) but now it has XP Pro installed on it and takes forever to boot up and open programs such as Firefox.
But I'm sure there is a reason for XPs slowness, its because its got more features...right? Well at least it looks better! XP is also much more easier/better to use and that dreaded 'blue screen' and 'Your program has performed an illegal action' no longer appears. In my opinion, 98SE is no comparison to XP!