Originally Posted by DoctorOhh
Why they are taking up so much cpu time is immaterial if you want to fix it go to Preferences - Behavior and change Job priority from Normal to Low then restart calibre. This will ensure your computer is responsive to you and the calibre-parallel processes will cede to you when you want to do something other than stare at the computer.
You can also control cpu cores via Preferences - Miscellaneous, but once you change Job priority you should be fine.
I could be wrong but it could be because the original source of the epub is word html. Word html leaves a lot of crap behind in the html coding and converting those also takes a lot longer than normal epubs. The count pages might be having a similar problem.