(That should've read "may take" ...)
Thanks for taking the time to reply everyone.
@theducks: That sounds very knowledgable, but as I am unfortunately not I only have a vague idea what you're saying. I take it, there's nothing much I can do, if I'm not into messing with boards, interrupts (remember those from WfW 3.11), slots, bios,etc. Incidentally I am on a Dell (6+ years running).
@ DoctorOhh: "There is no very low" Yeah, well that was just a way of phrasing that Calibre's low is obviously not low enough, at least for Dell & me. I will fine tune Avast to exempt the said directories. Ta.
@Peter and BetterRed: Yes, I did limit the number of parallel jobs to 1. I'll try the core setting too.