The code path graph has a bit too much extra hair to be comfortable to read, so I'm sorry I didn't get round to this earlier. You have to be a bit focussed on just one activity to cut down on the noise, that's why docs advise binding it to keystrokes
There seems to be too many calls to pixbuflist's
draw_cb() That's the consistent little hot spot away from the drawing arc. Some layout issue is probably making it redraw too much, and I think that's a bug proper along the lines of "pixbuflist: redraws too frequently". Perhaps if a list is too long for its container, redraw ping-ping happens...
Please can you report this bug on the tracker? When you do, please paste in a link to this thread, because those call graphs are golden. Thank you!
It would be useful to know if it spends a lot of time in there when you aren't drawing, or even moving the mouse, when the sidebars are visible.