After recently updating to Lightroom Classic CC version 7.2 (February 2018 release), you observe one or more of the following issues:
Lightroom Classic CC version 7.2
Windows and macOS
When Sync With Lightroom CC is on, Lightroom Classic CC performs a one time activity in the background that involves going through all the photos in your catalog. Lightroom Classic computes MD5 hashes on the raw files to make sync faster subsequently and also to identify duplicates when syncing with Lightroom CC across devices.
Recommended if you do not sync your photos with Lightroom CC across devices
At the upper-left corner of the Lightroom Classic interface, click the Identity Plate and pause the Sync With Lightroom CC option.
If you want to keep syncing your Lightroom Classic photos with Lightroom CC across devices and don’t want to wait until the one time background activity is complete, see Workaround 2: Disable pre-computation of MD5 hashes below.
Recommended if you want to keep syncing your photos with Lightroom CC across devices and don’t want to wait until the one time background activity is complete
The System Info dialog (Help > System Info) should display the config.lua flags entry as shown below:
DigestManager.dontCalculateDigest = true