is it the flippin' video ads? It makes my browser (Firefox) painfully slow to respond - when I right-click on a link it takes forever to come up with the little box of options, and it doesn't even manage to keep up with my typing - the text displayed is really delayed, I'm well into the next word usually. Switching from browser window to browser window has also become ridiculously slow.
When I don't have any windows with MN open, everything is just fine.