Updated to 7.3 today and now it's very slow

I've been waiting fo version 7.3 of Lightroom (not the Classic version) as I want to migrate my Apple Photos (not an old iPhotos) library. This feature is broken in 7.2. I downloaded 7.3 today and was at first pleased to see the migration succeed. However 6 hours into syncing 41,000 photos it has uploaded about 210. It is _unbelievably_ slow. Any tips very welcome.

I have a fast internet connection (both up and down), am very familiar with cloud syncing on other platforms and am 100% sure this is not a connectivity issue.

[I actually don't even want my Photos library in Lightroom (non Classic) I want it in Lightroom Classic but I'm not aware of a direct route to make that migration]

You don't mention exactly what is slow. How slow it is now and how slow it was before.  Also helpful would be to provide your Mac's specs including OS Version Number. 

Generally, unexplained slowness comes from installation/preference issues. I would recommend a clean reinstall and then, if not fixed, a preference file reset. 

Clean Lightroom Install Procedure

• Close Lightroom
• Restart the computer
• Use the Adobe Creative Cloud App to uninstall Lightroom
• Restart the computer
• Install Lightroom via the Creative Cloud App without launching any other programs.
• Restart the computer
• Launch Lightroom
• Wait 5 minutes
  
  

This procedure works for both Lightroom Classic and Lightroom Desktop. A preference file survives an uninstall/reinstall. 

If you are using Lightroom Desktop, it can change your local storage location. Please review this setting after resetting preferences. 

Reset Procedure:

1. Close Lightroom.

2. Hold down [Alt/Opt]+[Shift] while restarting Lightroom.

3. Overwrite the Preferences when prompted by the dialog.

4. Close Lightroom.

5. Restart Lightroom.