I can suggest using '-hardframedrop' option to get higher framerate. It will cause some artefacts on screen, but will allow you to get higher framerate. Both DIVX and XVID are MPEG4 ASP: http://en.wikipedia.org/wiki/MPEG-4_Part_2

The difference between them is that XVID uses B-frames when encoding by default and DIVX does not. A good thing about B-frames is that they can be skipped on decoding when cpu is not fast enough, so when you are watching XVID video, some percent of frames can be safely dropped and you will not even notice that. But if the video does not have B-frames, almost all the frames need to be decoded as every sequential frame depends on the previous one. The only exception are keyframes, but they only appear in video once in 10-15 seconds. So if you try to play video which is too heavy to decode, you will only hear audio and see keyframes as a slideshow. It is possible to force mplayer to skip decoding of some frames (-hardframedrop option), but the result is often not very pretty as reference frames get missing and correct decoding is impossible in this case.

To sum everything up, XVID is more friendly to CPU limited devices.

PS. I've done some additional video decoding optimizations which should speed up mplayer. Also mplayer 1.0rc2 was released upstream recently, I'll try to switch to it in the next few days and provide the next build for maemo.