Could you please let us know how many images are there in your library?
Also, what is the format of the images? Are they Raw or Jpeg?
I have a total of ~30,000 images. They are now almost all synced, but I am adding a large number of mp4 files, and these are the ones that have been very slow.
I am experiencing a similar issue. Migrated off of Aperture and went through all the hoops to get projects and albums ported properly to Lightroom Classic. Decided to give Lightroom CC a go so I converted the library again and started syncing. I have about 20k files, 180 GB in total, and I'm using Windows 10. I started several days ago and have left my computer running. It has uploaded 5k files as of now. My trial period of Lightroom CC will actually run out before the files are synced to the cloud.
Moreover, the files are not synced by descending capture date, so the cool picture I took yesterday won't be accessible from my phone until maybe two weeks time, if I leave my computer on 24/7. Massive fail!
As a comparison, I placed my Lightroom Classic library inside my Dropbox folder, and synced through that. It took less than one day to have the entire library synced, which is more in line with the network capacity.
My trial will unfortunately not be extended. It is already quite expensive compared to other cloud storage solutions, and if the files won't be transferred quickly, I lose the very reason I would use it in the first place, which is cross-device access to my pictures, for social media purposes etc.
It'll make much more sense for me to set up an external workflow with exports to a Google Photos directory for mobile access, and keep my Dropbox syncing the full album. I really wanted to like Lightroom CC, but no feature in the world can make up for unbearably slow syncing. Adobe, you need to get some engineers on this issue urgently. Please let me know if you need debugging/reproducing info, I'd be happy to provide that.
I reinstalled my computer from scratch two weeks ago, reinstalled LR Classic and LR CC. I have some 28k synced photos, I left my pc on 24/7 and LR Classic has been syncing ever since. In the meantime I imported some 120 photos into LR Classic and created an synced album with those pictures. In a few hours the album was visible in LR CC on my iPad. I edited the pictures on the iPad (light, crop, flags, stars). I did that two days ago. The edits were synced properly to the cloud as they are visible online and in LR CC on all devices I have, however LR Classic that holds the original RAW files still shows the unedited versions of the photos. My intention was to make the culling and basic edits in the comfort of my iPad and then edit the selected pictures in LR Classic and in Photoshop eventually and then use the Facebook, Instagram and Smugmug plugins in Classic to publish things automatically based on rating. This is absolutely not possible. Even if I wanted to export the edited pictures from LR CC, it is not possible, because in CC only the smart previews are available. There goes my workflow. Quite a mess, I have to say.
The interesting thing is that syncing in the opposite direction, from LR Classic to CC goes much, much faster and it has nothing to do with bandwidth as my ISP gives me the same speed down and up. Actually LR Classic keeps using some 1 Mbps of the bandwidth all the time while the number of "all synced photographs" keeps slightly varying around some 80% of the total final number of photos shown in LR CC (some 20k synced in LR Classic vs. some 28k in LR CC).
So the bonus question now is: when I now need to have things done and go ahead and do the culling (flags and stars) and editing again in LR Classic to be able to publish my photos before this painfully slow process is done, will the newer settings be overwritten by the older ones not yet synced, or not?
After the recent problems with speed, I am now really seriously considering to cancel my Adobe Subscription. The ongoing issues already took more time of my life than smoking cigarettes did and if Adobe does not do about that really fast, I will be happier without.