File Open dialog freezing on macOS 10.12
The issue seems to be associated with iCloud drive. Here are two steps to resolve.


I seem to have resolved the freezing dialog box problem by disabling iCloud Drive. I have been experiencing what seem to be exactly the same problems, even after two fresh Sierra installs. 
If you don't want to disable iCloud drive, use Finder-> Preferences-> Sidebar- Disable iCloud drive to  remove the iCloud option on the Finder sidebar.