Ok, I put all of my books on the reader again, and now there is no extra memory needed. The Kobo still digests the new books, but to me it seems faster than before when the internal memory was full (which seems logical). Also the long time until it started to process the data is now down to only about half a second.
Regarding the speed: The speed for looking up books is better than on the PRS-T1 now, but for browsing shelves, that still is slow. Around 750 shelves need 3 minutes to show.
Thus it works but with the slow shelf management isn't practical.
Thanks for all your help, Joachim