DHTML Menu too Slow for IE8

Hi there ...

Right now my company works since half a year with Drupal and managed to create a Book with a lot of children and submenu's beneath.
Until now there was no problem, but since a week there is a problem that Internet Explorer, especially IE8 and IE7 tested
throw an error, that a javascript runs too long.

When i disable the dhtml menu for that specific book, everything works great, meaning for me the access to all that submenues etc. is too heavy.
In Firefox everything works great, but as always IE suxx the hell.

Any idea how to come around this bug? Especially because I need DHTML for that book.

Greetings
Zewa