After adding a large number of files using the Add Data button in ArcGIS Pro, clicking the Add Data button once again results in a long time taken for the files and folders to appear in the Add Data window.
The problem cannot be reproduced with the current version of ArcGIS Pro (2.0). After adding all feature classes in the geodatabase to a new map, the next time you click the Add Data button the feature classes in the geodatabase that was previously used appears immediately.