I assume that calibre is reading metadata and checking that the file is a valid book. Hard to do without unpacking. Calibre also may unzip, modify and zip again when you save or convert books. Hard to avoid if the book is to be updated...
The easiest way to speed up the unzip/zip is to make sure that the tmp folder is on the fastest HDD you have. Even faster is to use a RAM disk for tmp. In Windows you have to install some 3rd party software. In Linux it is built in (tmpfs) and you just add a line to fstab.
Only install a RAM disk if you have plenty of RAM in your computer.
You can specify the tmp directory used by Calibre using an environment variable, if you don't want to use the default.
https://manual.calibre-ebook.com/cus...ment-variables