Windows 8 was just released to the public. Note the developers only had a few days to publish their applications prior to release. Windows 8 and RT have an all new OS engine that iTunes isn't developed for yet.
I don't understand where the issue is here. I'm running Windows 8 Professional and iTunes 11 is installed, but its performance is just as bad as it always was. Nothing new. If you're restrictly referring to installing it, it worked fine for me.