By way of introduction, an exemplary snapshot from current processes:
/lib/x86_64-linux-gnu/libexec/kf5/kioslave5 /usr/lib/x86_64-linux-gnu/qt5/plugins/kf5/kio/thumbnail.so thumbnail local:/run/user/1000/dolphiniybvSu.112.kioworker.socket
Especially in larger archives and on optical disks parallelization could be faster and therefore less annoying. I thought about CPU threads/2 at low priority (idle/best-effort). Do you think this makes sense too or are there reasons against it?