Sounds like a disk time issue. I wonder if 7 talks to the HDD more than the other OSs have. Also important to remember that this is still a beta and betas are supposed to be buggy

I would try (if you have another HDD) moving your page file to another HDD to see if that helps. I know that in order to make it boot faster, it doesn't load as much stuff up front as before. Also, if you are downloading via IE8, it could very well be that because the download window is a plugin of IE. I know that for me, IE8 isn't playing too nice and it gets really slow, like when typing, but the rest of the system remains perky.