Unfortunately, slow movements will not render smooth with javascript.. this will improve as browsers progress [also I noticed that it’s smoother when not using zoom in/out]. And probably you should keep the effect not going that slow (over 15 sec a frame )

However, if that does not work out, you can use a flash based ken burns slideshow (got a few in my portfolio too) and keep this in the alternate div of swfobject ( for non flash enabled devices )