[Biebel]

Thank you ever so much for taking your time and replying.
I tried to look it up but ended up in developer pages that were over my head.
So, sorry but I could not figure out how to follow your advise regarding the GPU.

I did manage to do what I said in previous message.
I exported entire library, including settings but re-importing that would not rule out any database or settings issues.
So, I saved to disk and uninstalled afterwards.
I've seen a few messages regarding Calibre's cleanup of registry but I'm not sure what the fuzz is about, I did not have to remove a lot entries.
After reboot, installed again (32bit to start with) and re-imported the saved directory.
Obviously, this took a while so I left it overnight. This morning I found it finished with only a few warnings on duplicates.
I restarted the program and it easily opened the database of ~58000 titles in ten seconds or so.
The issue with the viewer, both external and internal, was gone. The gui remained responsive.
Problem solved.

Remaining Unknowns

Plugins:
I will add the metadata crawlers later tonight and if that works and I feel adventurous the "quality check" and "duplicate finder" later on.
I cannot imagine they caused the issues since I tried the "no plugins" option but if the issues resurface after doing so I will let you know.

Database:
The backup directory from three weeks ago was ~254gb, I made a simple backup just using windows explorer.
Its database was ~160mb.
The directory used in Calibre (the one with the issues) was ~250gb.
Its database was 148mb but after troubleshooting (deleting a few thousand files) it ended up 152mb.
After some bewilderment I remembered that databases do not automatically shrink in size when you delete their content.

The recently exported directory with the "calibre-data" files, the "save to disk" directory and the current working directory are all ~118gb.
Obviously the former two do not contain a database that I could check without importing.
Adding the "saved to disk" books from directory left me with the original amount of 58000 titles yet the database was only 88mb.
So, somewhere in the process, I lost some weight in both files and database, half of it, to be exactly.
I'm not sure but I seem to recall reading somewhere in the documentation that Calibre stores metadata and covers in the database (which makes absolute sense).
When saving the files to disk Calibre would export that content in the opf, saving the cover in a jpg.
When reading the files back from disc, it would not need to store that content in the database.
That would explain at least some shrinkage but surely not half of the original database.
Especially not considering the size of the directory shrunk proportionally the same half.

Files:
At first glance it appears like there are less duplicate formats but I could not say for sure.
Since the original directory and the current one are not even remotely similar in structure, I don't know a smart way to verify their differences.
There must be some tool out there that can do this but after 10 minutes of googling I was not able to find it.
At these points in life I wish that I would possess basic programming skills in Python or any other language for that matter.
I'm sure it would be a simple routine to verify the existence of the contents of a previously saved 63000 lines long "mybooks.csv" in the current directory.
Merely the title or uuid and formats would be sufficient.
I tried to think of a way do this in simple DOS but the multiple formats kept tripping me.
I could not think of another way besides re-importing the 'offending' directory from file.
Since I do not want to mess up again, I will use another system to do so and check what I end up with.
I might post the files issue later in a suitable part of the forum if I don't manage to get my head around it.

GPU settings:
I never got around to trying them since, to my shame, I have to admit I'm not savvy enough to follow your simple instructions.
That's on me, not on you, I really appreciate you taking the time to answer my questions.
Updating the drivers from graphics card left me with horrible AMD bloatware that threatened me it would remove the updated drivers as well if I would uninstall it.
I found a minimal install and will use it somewhere in time. Messing with the graphic card always makes me a bit nervous.

Again, thank you very much.*