I just installed Internet Explorer 8 on my Windows XP laptop. From playing with COMC.com for even just a few minutes I have noticed that the website is noticably faster.
Not sure if this makes a difference, but I turned off the compatibility mode option. Check Out My Cards appears to work fine without compatibility mode enabled, but other sites might have issues with this.
I just had to give a quick plug for IE 8. Every impressed with the significantly improved performance.