solved!
this change basically solved my problem; no idea if Windows sync client now runs as fast as upload via web browser, but in every case in runs thru properly in a reasonable amount of time.
=> Very slow sync (for small files) - #13 by mooleshacat
The 3.883 files mentioned above were fully uploaded within 20 or 30 minutes!
No clue why this still needs to be added manually and isn’t part of the shipment … but
i am happy!