I rarely do a conversion from .avi to DVD compliant but AVS to DVD would be my first try. The reason is if set up correctly it will use the HCEnc eccoder.
One of the few complaints I have with AVS to DVD is it won't let you use all the "bells & whistles" HCEnc is capable of. There is a way to script with AviSynth where it could do that but that is beyond my knowledge level .
If that doesn't give you good results you could try DVDFlick.
I don't have convertxdvd but many think it is very good so maybe you just need to make the right settings.
On the speed my computer takes even a little longer with DVDRebuilder . On encoding DVD compliant to single layer. So if you are using an average dual processor computer you time is OK.
I understand a Sandy Bridge is much faster & the soon to be out Ivy Bridge will be even faster.