Apple - Capturing screenshots on macOS Sierra is very slow
On High Sierra I faced the same problem, which had Google Backup & Sync as reason of failure (as Robert answered before). Instead of completely quitting Google Drive & Sync, I could also solve this problem via:
- Go into System Preferences
- Extensions
- In the left sidebar, choose "Finder"
- Disable Google Drive Extension
On OS X Mojave, the problem is solved by disabling the "floating thumbnail feature" like this:
CMD + SHIFT + 5 -> Options -> [Uncheck] Show Floating Thumbnail
Enjoy!
I had the same problem. In my case, I stopped the Google Backup & Sync program and then the screenshots were captured quickly again. Try quitting Google Backup & Sync.