IE8 will perform very poorly with V6. We do not recommend using this browser unless there is absolutely no alternative. This is a completely outdated browser that is far less secure, less compliant with web standards, and much slower than any of the modern browsers available (IE9, IE10, Chrome, Firefox, Safari). Microsoft is also dropping all support for IE8 in April:
"After April 8, 2014," describes Microsoft, "there will be no new security updates, non-security hotfixes, free or paid assisted support options or online technical content updates."
I would highly recommend switching to another browser as soon as possible, not only for Clarizen but to protect your online security and improve your browsing experience.