Open/save file dialog takes a long time to open in Windows
Solution 1:
It could be a shell extension, but in the past when I've seen this issue it was due to the shell trying and failing (timing out) to read a drive. In the example I have it was an aging 3.5" floppy drive, but I doubt that's your issue in the iMac. Nevertheless, the possibility remains that it's an issue trying to read information about one or more storage devices. In the case of the floppy drive it was easy to diagnose because I could hear the drive grinding when it happened.