cheuk5, welcome to the forum.
Yes I have tried DVDX from the begining and although it is quick and relatively easy to use, the quality of the picture is quite poor compared to using TMPGEnc. Even using FlaskMPEG with the same Panasonic plugin gives a better quality picture.
I convert most DVD's in about 5hrs, not 20hrs so I would guess your CPU's speed is about a 400. Mine's a 1.2g Athlon