Sure, what would be the full command at the client ?
iperf3 -c SERVER.IP -t 60s -R ?
edit -R, Reverse mode, remote host 192.168.1.199 is sending:
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bandwidth
[ 4] 0.00-60.00 sec 1.34 GBytes 192 Mbits/sec sender
[ 4] 0.00-60.00 sec 1.34 GBytes 192 Mbits/sec receiver
Btw, i've noticed that the TX-Power @5GHz is only 20dB, it was 24dB before with 17.01 Snapshot.
And 22dB @2.4GHz and before it was Tx-Power: 24 dBm as well...
But it shouldn't make a big difference as im testing when i'm right next to the router.
I've also disabled my Guest AP's for the test...
edit: another problem, i can't select any other channel than 34, 40, 44 and 48 !!
With any other the wifi will not come up or it will switch to disabled after a few seconds...
It's really strange but i had problem as well with the 17.01 Snapshot build !
Here is my Wifi config @5GHz:
config wifi-device 'radio0'
option type 'mac80211'
option hwmode '11a'
option path 'pci0000:00/0000:00:00.0/0000:01:00.0'
option htmode 'VHT80'
option country '00'
option legacy_rates '0'
option channel '48'
config wifi-iface 'default_radio0'
option device 'radio0'
option network 'lan'
option mode 'ap'
option ssid 'SSID'
option encryption 'psk2+ccmp'
option key 'PW'
I've just tested 17.01 Snapshot (r3909) and it's about the same throughput..
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bandwidth
[ 4] 0.00-60.00 sec 1.69 GBytes 242 Mbits/sec sender
[ 4] 0.00-60.00 sec 1.69 GBytes 242 Mbits/sec receiver
Reverse mode, remote host 192.168.1.199 is sending:
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bandwidth
[ 4] 0.00-60.00 sec 1.64 GBytes 235 Mbits/sec sender
[ 4] 0.00-60.00 sec 1.64 GBytes 235 Mbits/sec receiver
It seems to be a limitation of my Intel AC8260 Wifi Adapter (Single Modular) as i get about the same Speed (~300Mbps) with my WRT3200acm @5GHz, (80MHz with Channel 100 and 27dBm).
The weird thing is that i've found a side which tested a notebook with the Intel AC8260 Adapter and they were getting way better speeds than i do...
Later i'm going to test a Dell Latitude 3590 with Qualcomm QCA61x4A Wifi Adapter and see if it makes a difference...
Mega @Kherby quote 
If the router is not doing NAT, it does not make sense.
Me too, so that is why.
I believe this is broken/not implemented in the mt76x2 driver so basically a meaningless number.
Both are 2x2 cards according to their specifications so the theoretical maximum is 876 MBit/s. However, you said your card is single modular aka one antenna? If that is the case the speed is halved.
@All, made a new testing build without compiler optimization flags to rule those out.
Well the test picture is the exact same Model which i own (Asus P4540UQ) so it should be 2x2 as well, but i don't reach those speeds for some reason.
On the back of my Notebook it says "Intel Dualband AC8260 Single Modular, Model Number 8260 NGW"
Imho i should achieve more than ~300mbps but i'm not sure...
I've uninstalled Intel Wireless Pro set and cleared the driver + software from the device manager as well and only installed the newest driver but that doesn't make any difference.
I'm really confused !
My Notebook was send to a Asus Repair center once (Touchpad wasn't working), so maybe they damaged something with the Wifi Adapter when they repaired my device ?!
Or maybe they forgot to connect one antenna. Or sometimes there is only one antenna to push costs down. What is the model of your laptop?
The same problems with my test build so there may be something rotten in my build environment. So I will set that up from scratch and see if that fixes it.
Yeah maybe they f***** up with my Wifi adapter... They also forgot to put one screw back in !! These damn Asus Service fools !! Asus service is really not the best, at least in my country... 
My model is: Asus PRO P4540UQ
And here is the test: https://www.notebookcheck.net/Asus-ASUSPRO-P4540UQ-7200U-940MX-Full-HD-Laptop-Review.211876.0.html
I haven't had the chance to test the Dell Notebook of my fiend yet, he went to the club this evening and i'm sitting here crying about my damn wifi !! 
Just flashed r7301, found the release doesn't include kmod-usb-storage and uas. I plugged in my USB3 HDD and didn't work.
Higher channels are DFS channels and require more than a minute before they come up. They need to scan for radar interference.
Ok i'll give it another try...
Here is iperf test with my friends Dell Notebook (Latitude 3590 @ Qualcomm QCA61x4A 802.11ac Dual Band Wireless-Adapter (Bluetooth 4.1, 2x2)):
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bandwidth
[ 4] 0.00-60.00 sec 1.29 GBytes 185 Mbits/sec sender
[ 4] 0.00-60.00 sec 1.29 GBytes 185 Mbits/sec receiver
Even worse than my Intel 8260 Adapter... I'm getting the same speeds when connected with my WRT3200ACM.
Maybe the cheated him as well and just built in a 1x1 Adapter, would be a shame as the datasheet clearly says 2x2.
Can i somehow verify 2x2 without opening the device ?
edit: this is what i see in Luci:
866.7 Mbit/s, 80MHz, VHT-MCS 9, VHT-NSS 2, Short GI
866.7 Mbit/s, 80MHz, VHT-MCS 9, VHT-NSS 2, Short GI
For 866.7 Mbit physical rate you need two spatial streams, so you are running 2x2
Well than i dont understand why the throughput is so low... imho it should be ~twice as much. 
Could be a lot of things, for example:
-multiple devices connected to the AP
-interference from other 5 GHz network
-distance to the AP
-orientation of the antenna of the AP
-driver not making 100% use of the hardware
-the placement of the antenna in your notebook
And more, wireless is a complicated beast
@Bartvz
I will try to test different channel aso..
The werid thing is that i reach the same speedlimits with my WRT3200acm @Channel 100 and i'm even using Linksys High Gain Antennas with this device... It's really confusing !
I've always found that the stated speeds are highly exaggerated and my solution is everything critical (even laptop, tv, htpc) on wires and the rest (wife's laptop, phones etc) wireless.
People tend to forget that wireless is not full-duplex but half-duplex. You cannot send and receive at the same time with 802.11. So if you see 866.7 Mbit/s, the real throughput is half of that. Still more than enough for most use cases. I only have a 10/1 Mb/s line and I do not pump crazy amounts of data in my LAN network. Wires will always be superior because they are full duplex and because they are not as complicated as the whole wireless thing is.
@All, redid my build environment and made a couple of test builds. Without or with compiler optimization flags does not seem to matter. Changing 5 GHz wireless channel from 149 to 100 to 36 saw some speed and stability increase. After a couple of iperf3 and speedtest runs, the 5 GHz wireless is still up. However speeds max out at around ±40 Mbits/sec which is just plain weird. Maybe something wrong with the configuration on my DIR-860L? A bit stumped at the moment.
Since it is hot in the Netherlands at the moment (26C), I attached an USB fan to my router to rule out overheating.
Edit: After 8 hours 5 GHz WiFi is still up.
After 4-5 days i would say Wifi @18.06 is stable but today i couldn't reach the GUI anymore "Out of memory GGC Error" or something like this...
Had to cold reboot the device to gain access again.
And btw, im still not able to select any other channel than 34, 40, 44 and 48 !!
Even after 30 minutes the Wifi Interface doesn't come up... If i select a different channel the Interface comes up for a few seconds but than goes down and stays down.
Does is work for you ?
have you changed the country ?
Nope, it's default 00-World.