RT-AC5300: Issues with slow wifi speeds on low control channels

[solo]

Hi folks,

I have an RT-AC5300 router running Merlin's latest FW 380.68_4, factory reset and reconfigured. Running 2 separate 5GHz streams, 5GHz-1 and 5GHz-2, identical setup for each.

5 GHz-1 is using the lower control channels (36,40,44,48) and is set on auto.
5 GHz-2 is using the higher control channels (149, 153, 157, 161, 165) and is set to 153.

Everything on 5GHz-1 is slow af. I tried manually setting every available channel, still slow. I get ~30 mbps on my iphone, macbook, ShieldTV when testing on 5GHz-1 (any channel). As soon as I switch that device to 5GHz-2 it comes alive and pulls 300mbps+ (channel 153).

TRX on the MBP on 5GHz-1 channel 48 is 878 mbps, RSSi -48 dBm, Noise -92 dBm, 80MHz width.

There are 2 neighbor users on 36, 4x40, 3x44, 2x48; 3 neighbor users on 149, 3x153, 4x157

Any ideas on how to troubleshoot this?

 

[solo]

Now it gets more odd. It appears that the nVidia ShieldTV drags everything down!
So currently, I have my iPhones, iPad, Macbook on 5GHz-1, and the Shield is on 5GHz-2.
When I woke up this morning I did a speed test on my iPhone and Macbook and it pulled 300+.
So I thought the problem above solved itself. Then I woke up the ShieldTV to watch Netflix.
For whatever reason I decided to retest my iPhone and Macbook - it was back to a ~30 mbps.
I did this exercise 5 more times. Every time I put the ShieldTV to sleep, everything on 5G-1 is fast.
As soon as I wake up the ShieldTV, everything on 5G-1 starts to crawl.

Totally bizarre. Any ideas, suggestions?

 

[solo]

Just placed an order for an ASUS RT-AC3200 which I will configure the exact same way as the RT-AC5300 (same network names passwords etc.). Would be interesting to see if the problem there persists.