There is no delay for me using FF 3.6 or Safari 4 on OS X. However, if I move "too quickly" from one top level menu item to another, nothing happens. Not sure why, but I'd start by getting rid of code catering to IE5 & IE5.5, such as xGetElementsByClassName (iirc even IE6 had this, and 7 definitely did). Using element.getElementsByClassName('class names') will be much more efficient, and I guess that if your code is too slow, the above problem can occur.
But since you seem to be using someone elses code, why not get code which is less 1999 (IE5 release) and more 2006 (IE5 life cycle over)?
And while you're at it, why not drop js from the solution alltogether. For example, googling for "nested menu css" (didn't even have to type the whole thing) gives this page as its first result. Sure, it still handles IE5 specifics, but without javascript. And since it also describes the process of developping this menu, it wouldn't surprise me if you could stop before the IE specific sections, since IE8 probably will be able to work with only the non-IE specific parts. If you need support for IE7, add the minimal extra stuff to make it work (if it doesn't actually allready do so).
