Come on! How often do you
add enough books to make FAST a significant need? You HAVE your Library. Adding books now is probably less than a dozen at a shot. (It takes me more time to download and UnZip a Baen Monthly Bundle, than Add it to Calibre and do my added housekeeping on those.
Converting is where the Horses are needed.

Start up Task Manager or Process Explorer. You can watch the various demands: CPU, RAM, I/O, Net
#1 Lots of RAM to give things room to happen without memory caching
#2 almost any modern CPU that is not rock bottom. This is not Gaming...You do not need the hottest i7 or AMD to run Calibre. an i3 should do the job.
#3 a decent HD. While SSD will get fast access to files... but other than startup... Calibre chugs along in mostly a single thread (a quad core does not gain a whole bunch here)