Ah ha!
this is why: in wired i checked it is 100 Mbp
specs @ https://eu.dlink.com/uk/en/products/dir-816l-wireless-ac750-cloud-router says wired is FAST ethernet, which is 100mbs.
a lot of 4k content peak way more than 100mbs.
see test results @ Reddit - Dive into anything
this is also explained in the 4k faq @ Plex, 4k, transcoding, and you
you need to either upgrade the router to one with gigabit speed, or buy a gigabit switch, plug shield and nas and existing router into it.
