To me the browser discussion has always been on standards, some additional buttons and open-source ideals. Rendering speed has never been an issue.
The other way around, the browser had never been a factor in the performance of my applications. I tested load times of a certain page or application in a single browser, no more than that.
A year ago this changed for the first time when it appeared that IE7 was dramatically faster in displaying InfoPath forms than IE6 (/koning53/archive/2007/10/15/avoiding-performance-issues-in-infopath-forms-servers.aspx). Since then I advice clients using Forms Services to upgrade to IE7 or to install the IE6 hotfix for javascript performance.
More recently I've been hearing end users complaining that SharePoint is utterly slow and unresponsive. Slow?! Per user we've got an average of 3 Xeon-Cores and 8gb RAM desperately waiting to spit out content to the network (we've got a small group of people testing our new server farm). Those pages should be flying by the speed of light from SQL server to the screen.
It turned out that 80% of the people where still running IE6. Last week I've done some rough estimates using a stopwatch, but apart from not being a real test setup, the results are quite shocking.
Load times for a `large' list-view page (350 items with 7 columns):
IE6: 34 seconds (this is a clean system, just installed, no other apps running)
IE7: 21 seconds (this is a VPC, so limited to 1 CPU core and limited on memory)
IE8 in IE7 mode: 6,4 seconds (the same system as the IE6 test)
IE8: 6 seconds (the same system as the IE6 test)
This morning I've put Safari 4 to the same test (on the same system), it passed the test in less than 4 seconds. On the other side, the rendering failed in many ways so this is no alternative. But it shows there is still space for improvement.
Conclusion? The browser has become a large factor in the user-experience for your SharePoint environment; the perception of the user can be totally determined by the speed of the browser. So instead investing heavily in more servers for more speed, try to investigate the other (cheaper) alternatives for performance.
koning53