So did a little testing. It's definitely the same process as the "Add folder dialog".
I have a couple of network drives, "E:" has >6k folders, "F:" has 700, and "G:" has 15. The pop up is instant for G:, ~20 seconds for "F:", and probably a few minutes for "E:" but I didn't time it.
All of them load instantly if I just navigate to the drives in the viewer. It's just that dialog box and the autocomplete popup that are slow. Though, the dialog box makes DOpus freeze until things are loaded.
@Jon said in the other thread that you are working to improve that in the next update?