OK. Your wifi spectrum in the room is disturbed somewhat.
RX rate is almost at the MAX speed (MCS08. See http://mcsindex.com/
In the TX rate the AP was not able to communicate with the 2 streams , only 1S. Is a sign of a distorted or disturbed signal (might have a too strong TX power set in the AP)
(-40 dBm is a very strong signal, -30 dBm can saturate a receiver)
Now you know where to look! And add the TX/RX CCQ column for the signal quality, should be 95% and above.
Rates only come after enough traffic has passed. Before that the numbers are very unpredictable.
I hope the 5th device improves with traffic. The signal strength is excellent, but the CCQ might be very low if it remains low. A slow device uses more "air time" and blocks others!
I don't know the Wifi Sweetspots APP, but if that Spot 1 is a real measurement, then this is a problem. -6 dBm is enormous, even dangerous to human health. 100000 times stronger than normal. Even at a different frequency it is likely to interfere. Please use "Freq Usage" and "Snooper" to check for the spectrum around. It is probably not a real signal measurement.
Out of the box, there are no mangle rules to change the priority, so A-MPDU for priority 0 is used all the time. Setting the others is OK. But should not be done for voice (6 -7) as this is not high volume but delay and jitter sensitive. WMM is needed to activate the priority queues. And own mangle rules are needed to set the priority, or all is priority zero !