In order to achieve optimal load performance with RadFileDialogs controls, you should be familiar with the following performance tips:
Whenever possible, you should use a single instance of a dialog for multiple openings. This will reduce the opening time for every open after the initial one.
Setting the ExpandToCurrentDirectory to False will improve the loading time.
Setting an InitialDirectory that has fewer directories and files will reduce the loading time.
When developing an application that contains RadFileDialogs, run it in Release mode, instead of in Debug mode. This way you will avoid the caught exceptions while traversing the file system, which will improve performance.