Hi,
We are aware of some slowness in the return of this dialog box and it's performance gets slower as your folder structure expands. We are currently looking at various ways to improve the performance of this.
One thing I'd like to check with you is how's your machine's performance when you are trying to display network folders? One thing you can try is in Windows Explorer, click on Tools -> Map Network Drives, click on the Browse button and try to expand the folder structure of any network folder which has a decent amount of sub folders.
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Kit
Ying Kit