I installed Internet Explorer 8 some days ago and wondered why the integrated Webbrowser now takes very long to start and why opening a MIME email takes more than a minute.

According to some tests with Process Monitor Lotus Notes 8.x Standard on Windows is using Internet Explorer. This IE checks the registry for restricted sites to decide if it is OK to execute code, allow cookies, etc.
Check these registry paths':

• HKCU\Software\Microsoft\CurrentVersion\Internet Settings\ZoneMap\Domains
• HKCU\Software\Microsoft\CurrentVersion\Internet Settings\ZoneMap\EscDomains
• HKCU\Software\Microsoft\CurrentVersion\Internet Settings\ZoneMap\Ranges

Usually you have only some domains listed as you have to add them manually but if you use Spybot Search and Destroy to immunize your PC you will see hundreds of domains listed.
As long as Microsoft does not fix the problem with IE8, bringing back the performance known from earlier IE releases, you can undo immunization.