not getting fast 802.11ac WiFi speeds

Question

I'm using a Linksys WRT AC1900 router and an Intel 7260 client adapter on my desktop. I've never seen speeds > 10MiB/s at my previous apartment, probably because the router was 30 feet away and the signal had to go through a door.

But now at my new place, the router is only 5 feet away, and the speed did go up to 25 MiB/s ~= 200Mbit/s. But this is still not at the theoretical speed of 800Mbit/s = 80MHz * Log2[1000] (Shannon channel capacity, only 1 spatial stream). I'm not complaining, since for everyday use, 200Mbit is plenty. This itch is more of a technical fetish I'm trying to scratch.

I have OpenWRT installed and am using iperf3 to test bandwidth and that's how I measured the 25 MiB/s.

In Windows network connections, I see the speed reported as 866 MBit/s

Likewise, I see the same report in OpenWRT:

This speed seems impossible for a 20MHz channel and -47 dB mW signal and -87 db mW noise (only 532 Mbit/s based on Shannon channel capacity and 2 spatial streams).

But that's a huge clue. It says my desktop is only connected at 20MHz, not 40 or 80 MHz. I played around with the OpenWRT settings and tried lowering the channel width from 80MHz to 40 MHz and got my desktop to connect at 40MHz (seems the Intel card doesn't recognize 80MHz), but the iperf3 benchmark didn't get any faster.

Can anyone help me with this mystery?

Possible problems:

1. OpenWRT doesn't support 40 or 80Mhz channels? I'm using the Chaos Calmer version of OpenWRT and this is the first release for the Linksys WRT 1900AC

2. Intel 7260 driver issue? I see a lot of complaints for this adapter on forums.

3. Bandwidth needed for 80 MHz taken by other routers? Doesn't seem likely since I used WifiInfoView to check.

4. iperf3 transmit size too small? It's well known that as bandwidth keeps growing while latency stays the same, the more important it is to use large writes to achieve high throughput. I did try increasing the buffer size from the default 128KiB to 1MiB (-l 1048576), but it didn't help. I also tried iperf3 over a wired link and got over 900 Mbit/s as expected.