I've been considering capping mine at 30 as well. I think the hardware steamlink sets about there..

I still find it strange. I will try to set it to 30. But, maybe I should use a cat6 cable? I got cat5e cables but I just don't know if they are good quality. I didn't find any program that test network speed. If someone knows one...

seems to be pretty smooth at 30

Tried it on "beautiful" at 30 Mbits and did get occasional momentary audio drops but otherwise pretty good.

Stable connection using ethernet from my router via powerline connection. Was getting something similar (speed test hits 95Mpbs, then stays at 90), but decided to check the actual network speed at my Pi 4, as I have no other way of verifying the speed through the powerline connection. iperf3 indicates my actual transfer speed varies between 83.5 and 106 Mbps, with an average of ~95.3Mbps.

I realize this certainly doesn't cover a case like pinut or Xenophobe's, but it might be worth testing your connection speed with a different tool to verify. The Steam Link diagnostic might not be too far off in my case :D

Just want to say this has helped for me too. Network test caps out at 95Mbits but around a second lag when playing games. Set a maximum of 30 on my Pi3B+ and all runs smoothly. Thanks for the advice!