Our system is using the MS ReportViewer.Webforms object to populate an ASP response page with the report contents. There is great variation in the javascript processing time between the various versions of IE. When we used the 2005 version of the reportviewer object. a report would render in about 20 seconds. When we switched to the 2010 version of the report viewer, IE8 rendering times jumped to 75 seconds. This is all using the same version of Windows and SQL Server. (2008r2 for both) I discovered that our system was causing IE8 to force IE5 emulation mode when rendering the report. Using IE7 emulation mode reduced the rendering time to 40 seconds, IE8 standards mode reduced this down to 30 seconds. These two pages helped me to sort out what was happening.

http://msdn.microsoft.com/en-us/library/cc288325(v=VS.85).aspx

http://msdn.microsoft.com/en-us/library/dd565628(VS.85).aspx#browsermodes