Gecko takes a long time to redraw widgets and page features. To get an idea of what I'm talking about, look at this screenshot:
This is what happens when you drag a pseudotransparent terminal across a Galeon (or Epiphany, or Firefox, or Seamonkey) window. It happens with any window manager. The tracking is quite easily visible; you don't have to know what to look for.
Also, window contents resize slowly and jerkily when a browser window is resized. This is especially visible with lighter window managers such as Fluxbox, in which it may take up to a second for the contents of a window to resize so they fill up the window properly after the frame is expanded.
It should in addition be noted that Gecko seems to increase the redraw times of other widgets. See here:
With windows other than a browser window being dragged over it, the Nautilus window has finished redrawing formerly-hidden widgets almost as soon as the trailing edge of the other window has passed over said widgets. With the Galeon window being dragged over it, it still has not redraw icons or menu entries at this point.
Perhaps more to the point, this is what the Galeon window looked like while it was being dragged about:
This can probably be partially blamed on Metacity's sluggishness, but still, other than in Gecko-based browsers, I have only ever observed such behavior in applications that uses Java Swing.
By comparison, the Linux port of Opera has significantly faster redraw times; Konqueror's redraws are not visible unless you look very carefully. This isn't to say that Gecko is not a good rendering engine - quite the contrary - but it does seem to me to have some redraw problems, at least on Linux with GTK, especially when using browsers that are compiled against XULRunner.
[Edit: Sorry about the original title of the thread, which is still visible in the screenshots. I decided that it was vague and might be percieved as trollish, but I didn't feel like redoing all the screenshots...]