Since that post, I took out the IE workaround and implemented balord's Prototype 1.6.0 suggestion as noted above. It is a significant improvement but still exhibits major slowness on IE. IE was still using 1 GB of RAM to display my webpage when scrolling!
So, in addition to this change, I converted some shadowed boxes to singular .png images. This reduces the amount of Javascript processing required by IE. Now, IE is using "only" 400 MB of RAM to display the page when scrolling. IE now scrolls better but I still consider the performance poor. It jumps from section to section when scrolling.
This is a lot of "workaround pain" to make iWeb pages more usable (but still less than ideal).
And, on top of all that, my pages are now broken when viewed with Firefox. For example, if you look at this page:
http://www.lititzsummershowcase.org/College_Coaches.html
you will find that some box shadows are missing (e.g. on top yellow box) on Firefox but appear just fine on IE and Safari. This new problem is not related to the Prototype 1.6.0 modification.