Encoding times often rely on the speed of your cpu. And DVDFab cuts a lot of corners in their blu ray encoding already, so I doubt you'll find a software solution to speed it up.
If you really want to speed things up, you'll need to invest in a newer computer with the Intel Sandybridge cpu, or the upcoming Ivybridge cpus.
They can make use of a technology called Intel Quick Sync, which is inferior in quality output to what can be done with the X264 encoder, but will probably be close to what DVDFab does. Quick Sync is much, much faster however.
DVDFab does seem to be able to make use of Nvidia's Cuda technology, so if you have an Nvidia video card that can use that, it should help as well. DVDFab also has something they call Lightning-Recoding, but I don't know much about that: http://www.dvdfab.com/lightning-recoding.htm