After recently updating to Lightroom Classic CC version 7.2 (February 2018 release), you observe one or more of the following issues:

• Editing metadata of multiple images is taking a long time to update.
• The keyboard shortcut key B to 'Add To Target Collection' or 'Remove From Target Collection' isn't working properly. 
• In the Develop module, the image selected in the Filmstrip is not loading in the Loupe view.
• Unexpected slowness or strange behavior.

Product version affected

Lightroom Classic CC version 7.2

Operating systems

Windows and macOS

Cause

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. 

Workaround 1: Turn off 'Sync with Lightroom CC'

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.

Workaround 2: Disable pre-computation of MD5 hashes

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

1. Launch Lightroom Classic CC.
2. Choose Edit > Preferences (Win) or Lightroom > Preferences (Mac).
3. In the Preferences dialog, go the Presets tab.
4. In the Presets tab, click the Show Lightroom Presets Folder button. The root preset folder opens in the File Explorer or Finder. 
5. Navigate to Lightroom folder.
6. Go to the link https://adobe.ly/2GV75fM and download the config.lua file. Copy the downloaded config.lua file into the Lightroom folder mentioned in the previous step.
7. Relaunch Lightroom Classic CC.

The System Info dialog (Help > System Info) should display the config.lua flags entry as shown below:

Config.lua flags:
DigestManager.dontCalculateDigest = true