WebKitWebProcess eats about 12 to 15% CPU when this site is opened, even when it is hidden: https://www.humblebundle.com/games/capcom-sega-psn
epiphany itself yo-yos between 5 and 12% CPU. The tab should be throttled down when hidden. Both processes' CPU usage drops to a couple of percent when that tab gets closed.
This was fixed by r242364, see bug #195270. We are now suspending the threaded compositor when rendering is suspended. I've noticed another problem, though. When resizing the window, the hidden tabs are updated too, to avoid flickering or getting the old size when switching tabs. For that we need to resume painting and we are not suspending it again after the update. I'll file a new bug for that.