performance - How can I simulate a slow machine in a VM?

I'm testing an AJAX-heavy web-application. I develop on a new Mac, but I use VmWare Fusion (currently 3.1.2) to test in Windows XP, using IETester to simulate older versions of IE.

This lets me see how older IE versions would render the site, but I'd also like to see how the site would perform on an older machine. I see in the VM's settings that I can decrease the RAM; is there a way to also dial down the processor speed? How else might I simulate a slow machine?

(I am also going to check out how to simulate a slow internet connection.)