…I had labouriosly linked the pages, so I thought that might be the problem.
yes, it is – a huge amount of text in one text chain is known to slow dowm scribus.
I started unlinking some pages, and the text disappears.
it is still there, but as an overflow in the first text chain (and therefore invisible…)
if you work with linked text frames, you should import one chapter (or other logical part, depending on the project) at a time, then you unlink the chain and import the next chapter into the following frame, etc.