Caveats
There are a couple of things to watch out for: Firstly, spacetime is only reported as a percentage, you have to compare it to the overall size of the program to decide if the amount of memory is worth pursuing. The graph, with its kilobyte vertical axis, is good for this.
Secondly, Massif only takes into account the memory used by your own program. Resources like pixmaps are stored in the X server and aren't considered by Massif. In the Swell Foop example we have actually only moved the memory consumption from client-side pixbufs to server-side pixmaps. Even though we cheated there are performance gains. Keeping the image data in the X server makes the graphics routines quicker and removes a lot of inter-process communication. Also, the pixmaps will be stored in a native graphics format which is often more compact than the 32-bit RGBA format used by gdk_pixbuf. To measure the effect of pixmaps, and other X resources use the xrestop program.