ConvertXtoDVD is usually regarded as one of the faster programs for conversion of various types of video to DVD-video. I believe they use ffmpeg as the encoder.
All other things considered equal, mpeg-2 conversion relies on the speed of your cpu. If you have a relatively slow computer, you're going to have slow conversions.
While there are a handful of good, free alternatives, like DVDStyler, DVDFlick, and AVStoDVD, most use ffmpeg. AVStoDVD can use the HC encoder and is generally slower, especially if you use it in the most recommended mode, which uses two passes.