I also have a Quad and NVIDIA Shields. Like you, I’ve also noticed less than Gigabit transfer speeds from my Quad on my CAT6 wired LAN. Thread here: Tablo Quad iperf3 performance not quite Gigabit?
I asked support why the throughput of iperf3 is reporting less than what it should be capable of. I haven’t gotten a definitive response on that yet. Kinda sucks for ripping. Thankfully it doesn’t impact actual viewing.