The good news is that there is no real problem in DOSBox; the bad news is that your image is effectively "broken" as far as DOSBox's IMGMOUNT system is concerned.

If you look at the files in the \KEEPER\DOSSETUP\DATA folder with the image mounted in Daemon Tools or Alcohol 120, you'll see a bunch of files with hyphens in their names. Look at the same files with the image mounted directly in DOSBox, and there are underscores instead of hyphens. The Dungeon Keeper install program uses hyphens when it looks for the files, and does not report an error when it fails to find them, thus the weird behavior with missing text and other problems.

The image you have was authored (or, more likely, REauthored) with software that added the Joliet extension to the ISO9660 file system. Windows prefers Joliet because it is less restrictive than ISO9660, so you see the files with hyphens when you MOUNT virtual drives. However, DOSBox's IMGMOUNT uses the ISO9660 filenames.

The original CD has hyphens in the ISO9660 filenames, which is actually a violation of the ISO9660 rule that says filenames should only have uppercase alphanumeric characters and underscores, but MSCDEX doesn't go out of its way to "fix" the filenames, so Dungeon Keeper gets away with it. When your image was authored/reauthored, the ISO9660 filenames were changed according to the rule, but the original filenames were preserved in the Joliet extension.

Well, the problem was fixed, but the explanation was really interesting. I could make the image again, no problem 😀

I didn't see any comment on meowser's question about partitioning DOSbox to exploit the performance potential of multicore machines. I would think that could solve the speed deficit problem, especially in applications that are graphics intensive? Any interest in this?

not much to gain ?