I think Adobe basically just performs lossless compression to the DNG file and maybe cuts the size of the embedded JPEG.
Have you compared DNG vs. X3F though? I haven't compared directly but a few sites seem to suggest there are some visible differences between the two. I shoot in 21:9 and with the DNGs I can't get to underlying RAW data in LR if I want to recompose. (Or don't know how) It is easy to do in SPP. Lens profiles have to be manually selected in LR too.
SPP is certainly not fast but make sure you turn on the High Speed Mode options as that does speed it up. I also uncheck the "Always open in Display Quality Priority Mode" and just accept not always checking every file at 100% before processing. That took me awhile. Batch processing is good too.
With the X3F I think the data is more raw so if Sigma altered the color matrix or whatever it would apply to X3Fs. With the DNGs that is baked in by that point.