What I'm referring to is the perceived speed at which things happen on screen. How quickly finder windows open after double clicking an icon. How quickly your new tab is created and available in your web browser. How quickly a window goes away after you click the close button. That sort of thing.
The hardware in Mac os every bit as fast, if not faster, than most PCs. The pure processing speed is usually better, too (for example, a Mac will usually complete a set of Photoshop actions faster than a similarly configured PC). But, in day-to-day use, that responsivness of the finder and basic window actions is important, and Mac OS X definitely lags behind Windows XP here.
To clarify, I'm a bag Mac fan, but one who uses both platforms every day. I'll say it again: anyone who denies this is Macstrubating. The Finder, for example is NOT particularly responsive when comapred to Windows Explorer.