I've been using it since 3.0b5 and haven't had any slowdown issues with it. If anything, it's been considerably faster and more system-friendly than FF2, even on my P4-2.8.
Yeah, I read in one of the developer's blogs it's about twice as fast as FF2, and like, 3 times as fast as IE. The speeds were like IE: 600ms to load a page, FF2 400ms for the same, and FF3 like, 220ms.
Can't find the page though, because I'm lazy, and i can't use the Awesome Bar because I saw that before I got FF3.
EDIT: I have one nitpick with FF3. The "Save As..." dialog box seems much less responsive than in FF2. If I were to click the Save As button for something, I could immediately name it and then hit enter, even before the dialog actually appeared, and it would instantly do that that once the dialog appeared, and then disappear. In FF3, the dialog takes longer to appear, and I have to wait till it does to rename the file. On top of that, hitting enter after renaming the file doesn't always seem to work. And on top of that, if your mouse has a different cursor, and the dialog pops up, your cursor, while over the dialog, will flash between the normal cursor and whatever it was when the dialog popped up.
