well, unless the dvd you're copying is less than 4.7 gigs, you can either remove things from the original disc to make it fit onto a standard 4.7 gig disc, or you have to transcode it (re-encode it) into a lower bitrate mpeg-2
since your source and destination files are both mpeg-2, it doesn't need to decode the file to YUV between source and destination, it can stay mpeg-2, so it doesn't take quite as long as it would to go from an uncompressed video file to mpeg-2 or from a different encoding (say divx) to mpeg-2, but it still has to reencode every frame of video, and then repackage it into a .vob (.vob is a single stream with the video, all audio tracks, and all subtitles)
i have a 3.6ghz, and i know with my processor, and no other programs running, it still takes about 30-45 minutes to transcode an entire disc to make it fit on a 4.7 gig disc (depending on how much it needs to be compressed)
nero will allow you to transcode files and do on-the-fly burning, but you wont be able to burn faster than 1x or 2.4x, and most likely even then, you're going to be bumping into your buffer protection quite a bit, which you really don't want to use for a dvd-video.