IE 10 and Flash (Win 8)

So IE 10 (in desktop mode) is choking on a whole mess of sites (all of which load on my Surface RT without a problem) -- either slow loads (or none) or the 'compatibility mode' flag. Tumblr won't load at all, part of Ars are crunked, etc. Lots of other major sites load fine. It might not even be a Flash issue, but I can't see any other logic (since at least some of the sites I know are trying to load Flash based ad tech). New machine (HP Omni), clean Win 8 install. No aftermarket AV.

Installed Last Pass and thought that might be it (but installed except for the FF plugin). Have Chrome and FF installed, changed my default browser.

Had anyone else come across behavior like this? I want to use IE full time because the font rendering is so much better than Chrome.