I also noticed this when I was using build 7000 and build 7022 (x86).. I found it was a driver issue though.. I WAS using the latest available Windows 7 Beta Package from ATI on both builds.. The gpu in my main PC is a Sapphire Radeon HD 3870 512MB PCI-Express.. So what I did to solve this problem was uninstall the Windows 7 Beta Driver Package and installed the latest available Windows Vista driver for my gpu.. and guess what.. problem solved, all the windows now open/close VERY smoothly..
The cpu in my main PC is an AMD ATHLON 64 X2 5400 (2.8GHZ) True Dual-Core.. I also switched out my ram too though.. I WAS using 2GB's of Crucial DDR2 667MHZ ram at first... but later got 4GB's of Corsair DDR2 800MHZ ram and it also seemed to help...