In the Notes 8 standard client emails are rendered by the currently installed Internet Explorer instead of using the Notes internal rendering engine (independently of the "default browser" setting). The advantage of using Internet Explorer is the improved quality of the rendering of MIME emails. See IBM's Technote on this subject.
However, this may cause some trouble when using Notes 8 and Internet Explorer 8. It happens occasionally that users complain about a massive performance loss when opening emails in Notes - 30 to 60 seconds, until the email is rendered.
Alex Carmack wrote a very interesting posting about what can cause this issue: My solution to slow opening emails
The slow MIME email rendering is caused by security programs (like the popular Spybot S&D). They add thousands of sites to the Internet Explorer's restricted zone. But it seems that not only Notes suffers from this but als other programs: Internet Explorer 8 issues
They also suggest installation of a patch for Internet Explorer: KB969897
The problem is caused by the "Immunize" function of Spybot, but it can be solved with some tuning in the settings:
A prior run of "Immunize" can't be undone by changing the settings only - by selecting only the marked lines and press Undo -, it is also needed to clear some sites in the Internet Explorer zones list (internet options/security tab) or some cleanup using RegEdit to delete entrys in following Registry keys:
Another workaround is to set the "Disable embedded browser for MIME mail" in Notes client's perferences, this disables the use of Internet Explorer and Notes will use the old internal rendering mechanism again:
After that has been applied the mails may look a little different, but at least it is much faster.
It is also possible to downgrade the Internet Explorer to version 7 as the above issue applies only to Internet Explorer 8.