I am with dwanthny
Calibre does grab a lot of CPU when it is doing something. Then again, it is doing a lot
Just sitting there (no jobs running) it blips to 1 or 2% (probably polling for a device being attached). It shold not be pegged out when idle.
An overly aggressive A/V program that scans every file in use will bog things down.
I exclude my Library from active scans: The books were scanned when I imported/downloaded them and still get scanned on a Full scan.