Hmmm, yes. I have standard, up-to-date Windows Defender from WIN10.
I stopped my antivirus (real-time, cloud, and manupulation protection) , re-enabled EPUB for '
Use internal viewer for', and tried opening some books. They all opend very fast.
Ok, so then, one by one I restarted the protection processes, and each time tried opening some books again. With success!
To be sure I restarted the system and tried once more. Everything opens fast.
Perhaps some protection process was preventing some calibre cache to be cleared or something? Strange.
I'll leave the setting as it is now and will come back here if/when the problem might re-emerge...