IE 8 is slow, but that's not a real issue for most of the normal SharePoint users. However, recently I got a headache because of that.
There is a InfoPath 2010 web form which contains around 20 Rich Text Box controls. Most of them are usually empty, but sometimes users need to put more than 10 lines of plain text into the form, and they also need to print out the whole form with all data.
Initially Text Box control is chose. But if we set the height of each Text Box as 3 lines, then when users input 10 lines to text, only the first 3 lines are print out with the form. Rich Text Box control works well, however it's extremely slow (more than 1 minute) to load the form, even if the form is empty!
When I try to test the same form through Chrome 6 and FireFox 3.6, I guess you already know the result: it's lightning fast (form load in less than 2 seconds)! What we can do now? We cannot force all users turn to Firefox.
In the end, we have to use Text Box control for most of the fields in that form, and only use Rich Text Box control for the fields really need it.
Any way, with the release of IE9 next year (2011), it won't be an issue any more.