On one machine, I installed the IE8 beta. It slowed my machine to a crawl.
One must remember that IE is part of the Windows OS. When one launches IE, one is calling up a collection of files, ,dll's, etc., present and also involved in other processes.
So, IE is like no other "program". Everything one does can be affected by IE.
This machine absolutely slowed to a crawl. Its most important work now is to process data for BOINC projects and WCG projects. It also serves as a secondary repository for my 41.5 gigs of music, and I do use it to play music.
If anyone else has any experience with IE8, we would all like to know about it.