Changing Display Performance does not improve the speed.
However, I've just discovered that what is slowing down the performance is that the layout is set in two columns – after converting everything to one column, it's working normally (but it should remain two columns, so changing this setting is no option). Also, I'm working with the Paragraph Composer (but changing to Single-line Composer does not improve performance).
Nothing is linked to text files. But the text frames are linked, yes.
If I leave the files unchanged, working on half a dozen pages takes me one hour. There are almost one thousand pages ...