Usually we don't talk about performance in the doc, and in my personal experience I didn't notice any major difference between the different implementations (but than again I haven't used them much).
Talking about the other implementations and their advantages/disadvantages is fine, but things like "MiniDOM has a several times larger memory footprint" seems like FUD to me (see also http://docs.python.org/dev/documenting/style.html#affirmative-tone).