Slow running script in IE8
The tough thing here is that IE 8 is notoriously poor in terms of JS performance (compared to recent versions of FireFox, Chrome, Safari and even IE), and there's not a whole lot you can do to fix that.

However, in this case, I would consider one of two options:


Try to use different (faster) selectors besides :input[name="field_name[und]"] with Drupal's AJAX form states. If fields have IDs (they should, if the form is generated by Drupal), use the id instead, like field_name.
(If the above doesn't work) Try coding the visibility yourself (forgoing Drupal's FAPI #states system). You can optimize selectors and overall memory usage and performance by hand-tuning the jQuery or JavaScript yourself.


I'm guessing following #1 could fix your problem, though. For more reading on jQuery selector performance, see:


Your jQuery: Now with 67% Less Suck
jQuery Selector Performance Testing


However, for a form as large as yours, even hand tuning the JS might not be enough to get rid of that warning in IE. I have one form on a site I work on that always pops up that warning, and we sadly just have to tell users to ignore it. It's annoying, but it's acceptable to tell users to dismiss the warning in this particular case.