Found out that it seems to be the drawing of the progress bar, as weird as that may sound. When I switch to another tab, the CPU usage drops. I'm on Linux using the open source ATI radeon driver, that might have something to do with it.
Well that's quite interesting... Try and see if it still happens with fglrx. The progress bar is a CSS3 animation, which means Chromium likely hardware accelerates it, so I could imagine buggy drivers causing issues.
