Thanks for the detailed thread.
I think the problem is the lack of space on your Phone.Check how much space is showing up as available in Lr App.
It is expected from Lr mobile to take a maximum 10 secs to sync each image as it is syncing the Full Res DNG(if opted).
Also moving this thread to Lightroom for mobile & Apple TV
1 person found this helpful
I have the same issue with same version and app. I've managed to speed up the sync from ZERO transfer rate to pretty consistent flickering of Syncing/Pending status on the iPhone LR.
What I did was to go to the iPhone LR and press on the "Lr" logo at the top left of the screen. It will take you to a screen with your name, email, and the status of subscription.
Then just scroll down on that page to the near end. Press on "Clear Cache"
This made a huge difference. But out of 160 image needing to upload, I had to clear the cache 3 times whenever it began to slow down.
I hope it can help your situation.