Good time of the day,

I've recently turned my Archer T4U Plus into Wi-Fi hotspot using hostapd and opensource RTL8812BU driver on Ubuntu 20.04 and measured link bandwidth with iperf3. I've got the following numbers:

1) 50 cm distance, no obstacles

[  5] local 192.168.28.1 port 6666 connected to 192.168.28.100 port 58036
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec  7.89 MBytes  66.2 Mbits/sec
[  5]   1.00-2.00   sec  7.52 MBytes  63.1 Mbits/sec
[  5]   2.00-3.00   sec  8.26 MBytes  69.3 Mbits/sec
[  5]   3.00-4.00   sec  7.75 MBytes  65.0 Mbits/sec
[  5]   4.00-5.00   sec  7.79 MBytes  65.4 Mbits/sec
[  5]   5.00-6.00   sec  10.4 MBytes  87.5 Mbits/sec
[  5]   6.00-7.00   sec  11.5 MBytes  96.4 Mbits/sec
[  5]   7.00-8.00   sec  11.9 MBytes  99.5 Mbits/sec
[  5]   8.00-9.00   sec  12.2 MBytes   102 Mbits/sec
[  5]   9.00-10.00  sec  12.1 MBytes   101 Mbits/sec
[  5]  10.00-10.05  sec   766 KBytes   116 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-10.05  sec  98.0 MBytes  81.8 Mbits/sec                  receiver
 

2) 5 m, 1 wall

Accepted connection from 192.168.28.100, port 58050
[  5] local 192.168.28.1 port 6666 connected to 192.168.28.100 port 58052
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec   826 KBytes  6.76 Mbits/sec
[  5]   1.00-2.00   sec   665 KBytes  5.44 Mbits/sec
[  5]   2.00-3.00   sec   164 KBytes  1.34 Mbits/sec
[  5]   3.00-4.00   sec   334 KBytes  2.73 Mbits/sec
[  5]   4.00-5.00   sec   202 KBytes  1.66 Mbits/sec
[  5]   5.00-6.00   sec   345 KBytes  2.83 Mbits/sec
[  5]   6.00-7.00   sec   286 KBytes  2.34 Mbits/sec
[  5]   7.00-8.00   sec   112 KBytes   915 Kbits/sec
[  5]   8.00-9.00   sec   229 KBytes  1.88 Mbits/sec
[  5]   9.00-10.00  sec   544 KBytes  4.46 Mbits/sec
[  5]  10.00-10.07  sec  24.0 KBytes  2.74 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-10.07  sec  3.64 MBytes  3.03 Mbits/sec                  receiver

As you can see, there is a huge degradation in connection speed, and transfer rate varies greatly in both cases. The cheap 802.11n Keenetic router performs much better than that (I have 50 Mbit/sec transfer rate in the second case, with little to no variance). Why is that? It doesn't seem to be software bug, but rather something related to radio interface layer. I don't have enough knowledge of Wi-Fi tech to guess, but such issues aren't specific to T4U adapter - my tl-wn822n behaves the same way (802.11g mode)