The loading time of FileExplorer (the control used for listing files and folders in the Editor's ImageManager) depends on the count of the subfolders inside the currently loaded folder. The current behavior of RadFileExplorer is to call ResolveRootDirectoryAsTree method for each subfolder of the one that is currently requested to check if they have content, so decreasing this count should improve the loading time.
Another possible option you could try is to remove the TreeView from the visible controls of FileExplorer in case this would be acceptable for your scenario. Furthermore, if you choose this option you can optimize the performance even more by enabling VirtualScrolling for the RadGrid component in RadFileExplorer like explained in the following KB:
Setup virtual scrolling in the RadGrid embedded in RadFileExplorer
On a site note, we have planned improvements for the control in scenarios with large amount of files / folders, but unfortunately, I am not able to provide firm estimate for when it will be implemented (if possible).
Hope this information will be helpful for you.
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