When I do it I can see the URL cycling at my keyboard repeat speed (~30 fps), but the selected tab and page displayed in the foreground refresh about an order of magnitude more slowly. (1-5 fps) (So with a dozen tabs open, the effect is to select/show tabs in essentially a random order.)

edit: FF cycles smoothly (although with a weird white strobing effect with the main blank content area) if I open a bunch of blank tabs, but not if they're actual web pages.