Thanks for the dumps! Unfortunately, it looks like nothing much was happening in the process at the time they were taken. All of our threads were idle and waiting for user input. (There are some other threads created by Windows, but most of those look the same, and none stand out as meaningful.)
If you want to, a Process Monitor log might reveal a filesystem operation that is taking a long time: Process Monitor instructions
Looking at the screenshots, the layout is opening into a folder which has a large archive and a large installer/exe in it. Those could trigger lengthy virus scans when Opus inspects the files or asks for their icons, which can freeze the process (depending on how the antivirus scanner works). I wonder if you would see the same issue if the layout pointed to different/empty folders, or if any archives/exe files were moved out of the starting folders.
That wouldn't explain why it only seems to happen if the viewer is showing a JPG, though. That aspect is a mystery to me as it should not affect anything.