It would be interesting to know if the slowdown happens if you run from source. I realize that this is a rather heavy lift, but doing it would help eliminate several possible problem areas.
The complete instructions for running from source are at
Setting up a calibre development environment. One synopsis assuming Windows 10:
- Download the "calibre source tarball". Or if you want to be able to run not-yet-released calibre source, install "git" and "clone" the source repository.
- If needed, download a program that can extract files from a tarball. One example is 7zip.
- Use the extractor to unpack the tarball, storing the results in a folder you can easily find, best with no spaces in the entire folder path.
- Set a system or user environment variable CALIBRE_DEVELOP_FROM to refer to the folder containing the unpacked tarball. For more information, see this section of the calibre manual.
- Run calibre. If you are running from source you will see an asterisk next to the calibre version number at the bottom left of the calibre window.
Another thing to try both with and without running from source is running calibre in debug mode. Start calibre, click the down-arrow next to Preferences, and choose "Restart in debug mode".