DVD Shrink is not burning program, it just prepares ISO, and use other programs to do the actual burning. You tell DVD Shrink what to do after the ISO has been generated: Nothing, use Nero, use DVD Decrypter. I use None, and manually burn with DVD Decrypter later. Reason? I mount the ISO first in Daemon tools and see If I like it.
As for the compression part. Try once to have a Single Layer DVD-Video, or re-author movie only that needs no compression. This process will not use transcoding, so all the time spent is basically disk access, to read the files from VIDEO_TS folder and to write them in ISO package. This stage can only be improved with faster disk access, or best if you have 2 disks, one as source, one as target. Next if you transcode at say 90% and note the time, you can see how long it took to compress, as you already can estimate the ISO generation time. Transcoding is dependent on CPU muscle, the more the better, and there is never enough