Well, that was a slightly stressful experience
.
Rebooted my PC and it sat at "Configuring Windows Updates: 100% Complete" for over half an hour
. Presumably deleting all the files that cleanup had marked for deletion. It reduced the number of files in "catroot" from 4500 to 2500.
Has it made a difference? Difficult to say. Calibre's startup time from clicking the icon to the main window appearing was 45s, compared to 1m 15s before I did the cleanup, but the freeze time varies considerably from day to day, so it'll take me a while to find out. It looks as if it may perhaps have shortened the freeze, but it hasn't eliminated it.
Anyway, many thanks for the suggestion, chaley!