FTV 2 slow NFS speed on 5G wifi with 80 mhz width

Hello guys,
I upgraded today to a Wifi AC capable router (Asus RT-AC68U) to improve my transfer speed to my FTV 2 Box.

Im using the 5G Band with 80mhz channel width. The speed test show´s some impressive transfer rates at about 200+ mbps to the internet.
But there´s a strange behavior. When i start streaming a high bitrate movie (4K 60GB) from my NAS over NFS, it stutters and the buffer runs empty.
When i change the channel width to 40mhz, it runs smooth, but the internet speed drops to about 150mbps.

So why is 80mhz channel width slowing down only NFS speed. Internet is still lightening fast!?!?

I tested different channels: 36, 38, 40, 44, 48. No difference, only changing to 40mhz does the trick with NFS.
What´s going on here?! Any ideas?