Thank you! I was beginning to think that it must be a problem with my specific installation.
The only solution I have been able to find is to disable the compression. If I then close ComicBase and run SIdekick, I can at least save the backup online (as it does whatever compression is required for that okay). However, the compressed local file backups are never created.
I never thought of the local file creation. If I create a WinZip file, it is pretty fast. If I manually create a 7z compressed file, it works and completes, but takes about 10 minutes - still an improvement.
I must admit that the technically/programming elements are but beyond me, but hopefully others will have some ideas