@dentaku:

I tried both

IE9 32Bits and IE9 64Bits on my Win7 64Bits. All looks fine. But, today, the server is very slow. I also tried F12 using different IE7, IE8, and different document mode including Quirks Mode. All looks fine.

The site's performance is unstable, that's for sure.

I think there is something wrong with their JS. After I disable JS on IE9, it is back to 3 seconds load time.

How about show us the source code of the page?