Quick post to let you know that Internet Explorer 8 is causing slow rendering problems in Notes clients, at least with version 8.0 and 8.5. And by slow, I mean up to 50 seconds to render any HTML email. Have not tested with any other Notes client versions.

If you revert back to IE7, all is normal.

I did find a these entries at notes.net:
link
link

No firm information as to the exact cause, but others are experiencing the issue, including one of my customers.

I am not aware of any configuration settings in Windows, IE, or Notes to effect this. Currently, the recourse is to revert back to IE7.

So, look out for any IE8 installs!