The File Open dialog seems to get progressively slower after the first time it is launched. I’m doing it from the Import Assets button to pull an image from disk, but have also seen it on the Project -> Import -> BMML mockups menu.
To reproduce, open Balsamiq, go to Assets, click Import Asset. (It opens immediately), cancel it, and then click Import Asset again. This time it takes 15-20 seconds. Click Import Assets once more, and it doesn’t come back for around 2 minutes.
The directory does have 1300 icon images.