Hhm… when I think twice it must be your CPU that is too weak. On the ratDVD Homepage there are no minimum system specs … I have got another Computer in another town with an Athlon @ 900 MHz, 768MB SD RAM and a GeFORCE 3 with 64 MB… I would try it there but I don’t know if I get there before August the 31st.
Playback of ratDVD on my machine stutters. XVID files play fine.
ratDVD uses a lot more processor power than XVID due to its higher compression ratio. Although different, it can probably be best compared to H.264 with all options enabled - both in speed and in quality.
One thing that slows down the playback is the scaling done by the CPU. With the ratDVD settings program you can turn this of and let it do the graphics card. Depending on your card, it may look better or worse.
There will be more things possible in speed optimization in the future…