I have the ADFS scenarios depicted on http://technet.microsoft.com/en-us/library/cc779508(WS.10).aspx in the two sections both titled "Authenticating the user". When it comes to the steps 3 and 6 in the picture of the first "Authenticating the user" section and steps 5 and 7 in the picture of the second "Authenticating the user" section Internet explorer 8 causes 100% CPU load on one thread. The time it takes is proportional to the single thread processing power of the client CPU. On "old" clients it lasts so long that the requests fail due to timeouts. Even on modern clients 30 seconds delay occur. This occurs regardless of the operating system IE8 runs on. It works fine with Internet Explorer 6 and 7 and the latest public Platform Preview (Platform Preview 9.0.7745.6019/1.9.7745.6019) all perform well. I have tested this on several systems with clean installs of Windows XP, Vista, 7 and Internet Explorer 6 to 9 Preview in all possible combinations. Other browsers perform fast with this as well. I think Microsofts Single sign on solution Active Directory Federation Services should be compatible with At the moment this is not the case. IE8 is Microsoft latest browser and Microsoft shouldn't moan about slow enterprise adoption of new technologies if enterprise products perform so bad. This needs to be fixed in IE8 asap! Even with IE9 Microsoft still has to support 2003 and XP clients and IE9 won't run on these! Kind regards.