OK... disregard the entire above post.
I kept thinking that the issue was resolved but it is not. Paint.net on my computer takes 5 minutes to open an image THE FIRST TIME it is executed.
If I wait the 5 minutes (during which time the computer is entirely unresponsive, although keystrokes are saved and executed after the 5 minute waiting period), then close Paint.net and then open the same, or a different image, it fires right up as it should.
This issue is deeper than I initially thought and must relate more to other software/drivers on the computer than to Paint.net. I'll have to trouble-shoot this another day because my brain can't wrap around the problem now. Some weeks ago I entirely uninstalled Paint.net and removed its registry entries, re-booted and re-installed it only to suffer the identical issue.. Ho hum. If anybody has any hints in the meantime, feel free to comment.
Cheers.