Many thanks, brookter.
Sorry, I am not quite sure to understand correctly. The .docx file is the original file, the file I worked with, after importing it I noticed one cannot work with it in Scrivener, because it is extremely sluggish, so I saved the original docx file as rtf (with Word) and imported the rtf file (from Scrivener), but it has the same behaviour.
So import the docx file again? OK…hmm…what is the difference with a second import? I had imported that file a few times before.
Whoops, I would not have thought that…and I would have thought, 280 pages are rather a small amount of pages. With how many pages / slices can Scrivener cope? (until it gets sluggish)? Suppose, the output would be 1000 pages, could Scrivener manage them, were the editable in Scrivener without sluggishness?
So how can one get text being already written to Scrivener then? May be with that import split option, but where / how should the text / the doc be splitted?
I first tried it with a line break (just for testing), but Scrivener crashed after some time, too many splittings, obviously:

After opening it again, it shows a folder containing the slices, the folder is not accessible, the slices are, after clicking that folder Scrivener obviously freezed, so I closed it with the Task Manager.
Going through the original docx file? And with or without selections as title? Where can I find those options?
I have imported the doc with that option:

Leaving it like it is:

And the doc then was imported splitted in about 20 slices. When I click the folder (containing the slices) and scroll through the document (or the gathered slices) there is partially shown white space (I have to press the image down / up key 10, 20, 40 times to get to the next text) like:

When I click, seeing the blank area, “Reveal in Binder” a slice / text in Binder is marked and when I click it text (of this slice) is shown. Why is that? So, why does Scrivener show a blank area in the “folder view”, (so after clicking the folder containing the slices), but show text after clicking “Reveal in Binder”?
After importing such a big document to Scrivener into slices, how could I (automatically, not manually) verify that no text is missing, that each word is really imported?
Next try: I have added “###” to different positions in the original docx and imported it:

So it is splitted now in about 10 slices / documents. And it appears to be not being sluggish anymore. Again there is displayed a huge blank area, revealing it / different parts of the area shows slices containing text. What could that mean?
In the folder view (or whatever it is called, I guess “document” or “text”?) there is a lot of text missing / not displayed which is available - at least partially have not checked it yet - in the view you have clicking a document / item contained in the folder.
After compiling this message is shown when trying to open the compiled docx file in Word:

As an rtf file it can be opened.
Very many thanks!