Window resize in the Finder is perfect on my dual800G4, G4/400, and TiBook 500. Some applications, like IE and OmniWeb still have slightly slower Window resizing, but I think that's more the fault of the developer of the applications. Some applications display so much information in their windows (web browsers for instance), that live window resizing is going to suffer from the content in it. These developers (at least those with Carbon applications) can definitely make their app just use outlines when resizing, which is much faster (look at Mozilla and Appleworks, both use outlines when resizing).
Live application window resize is a very nice feature, but it's one that requires ample processor power to pull it off. I'm surprised that MS implemented it in IE, since they don't use it in Office v. X.
I may be wrong, but I think that Cocoa apps have to use live window resizing. I've yet to see a Cocoa app that uses outline resizing (Java apps use the outline method as well)...