Yes, RDC is active. So far no delay... luckily.
It turns out that some cable swaps - I haven't changed te UTP cables/bought new ons - to different GBit ports on the router.
Tomorrow is more a testing day in real life - also with my notebook.
I post my findings in this thread tomorrow.
I guess the testing and your suggestions did work in the troubleshooting. Thx for that... so far.