FYI, javascript and AJAX execute in the user's browser, not on the server (well, with AJAX the script that the browser is calling is obviously executed), so you don't generally see server resource increases from such functionality.
That said, IPB 3.0 is likely going to be a little heavier than 2.x, mainly because there's so much new functionality and much more abstracted code. You will definitely see an increased load at first if your site is open, as spiders are following new urls and reindexing content and so forth. It should level out pretty well over time, however. On the whole, after the optimizations we've done to 3.0, I think things are running about the same for 3.x compared to 2.x here.