Unfortunately, I don't have an answer for you. Your issue has a lot of similarities to what I was having (editor apparently not opening but using 10% of the CPU and also viewer taking a long time to open (both sporadically and randomly)). But, in my case, reverting my AMD graphics driver to a much earlier version fixed it (and that's weird because it was my AMD 5600X CPU that was ramping up).
Just grasping at straws here, but I've heard that some games used to have issues with too many CPU cores. AMD built a Game Mode into Ryzen Master to disable some of those cores so games would play better. You could try that out, but I've also read about people with Windows 11 not being able to install Ryzen Master. I'll put the link in here anyway if you want to give it a try:
https://www.amd.com/en/technologies/ryzen-master
The download link is near the bottom of that page.
Another wild guess might be to set core affinities for Calibre. But, besides knowing those words, I'm clueless about how to do it.
Sorry. Hopefully, someone else can give you some pointers.
Also, that's a nice looking system you've got there.