- Since I followed KG's advice and set
Behaviour->Job Priority from Normal to Low;
Miscellaneous->Max. simultaneous... jobs from 3 to 1 and
Miscellaneous->Limit the max. simultaneous ... cores from checked to unchecked (my addition).
the calibre-parallel process doing the conversion rarely gets above 10% of one core. And on my rig doing bulk conversions serially gives better throughput than doing them in parallel.