I might be wrong, but I think maybe some of us are getting a little overanxious trying to migrate everything to the cloud. I haven't done that and I don't intend to do that. I just tried uploading three DNG files to Lightroom CC. One of them was 20 MB, the other two were panorama images, each were about 135 MB each. The 20 MB DNG file uploaded to Lightroom CC (the new one) in the normal time. The two panorama files took about 90 seconds each to upload. But all three DNG files have been uploaded to Lightroom CC. And they were also placed in my designated folder on my hard drive, dated the date the images were taken. So I don't know what the issue is with your DNG files. Lightroom CC does seem to support DNG files. Just for your information, my DNG files were exported from Lightroom Classic CC and were generated from originals that were NEF files.
To Lightroom CC there is no difference between raw and dng files. They are simply a filetype it understands. It is possible that they were simply uploaded quicker as they are smaller than the original raw files in general.
It's just very odd that the Adobe preferred format is DNG, I had files in DNG in lightroom, but being conservative hadn't deleted the corresponding NEFs, and now the 'local download' cache of my adobe cloud images are all back to NEFs again, which is not what I want.
In fact sync is still running after 3 weeks with 20 Mbps upload speed - at full speed it would take about 2.5 days only, so something is very wrong.