Updated to 8.5 and had very poor performance in opening emails. It was the worse for HTML based emails. After a bunch of research, I used Procmon from Microsoft’s sysinternals utilities and found that I was running through the list of restricted web sites that is set in Internet Explorer. I cleared the 1000 or so sites listed in the list via the registry and my email is moving much faster. Many spyware products will add known sites to the restricted list and it looked like that Notes was reviewing each one during opening email.
Don’t know if this will help anyone else, but figured I would pass it along.