Hi,
I've been trying to optimize my app (using prototype151/scriptaculous), which works significantly slower on IE (no xpath) than on other browsers i tested
i've found that method getElementsByClassName causes extending every single element of the dom tree with Element methods, what seems quite costly especially for document node
that is because hasClassName method calls $ function on every argument and that causes extending (so we have 2 more func. for every dom node)
I suggest changing hasClassName to only call $ function when the typeof argument is string (attatched patch) - that gave me ~30% time save (IE) on initialization in my app