I am experiencing something very similar.
I have two SXT AC units about a 1km apart. RF stats are healthy, I think (-49db signal level, ~53db SNR), and CCQs are in excess of 90% during testing. My link shows hitting similar data rates as the original poster, about 400mbps or higher.
However, UDP speed test in either direction never exceeds 54mbps; that alone is suspicious, I think. It climbs right up to 54mbps and then never goes above it, in fact. TCP is about half that at best.
That test is done using the built-in Mikrotik bandwith test, but it mirrors what I experience doing speed tests or downloading large files. When I bypass the radio link and connect directly at the uplink side, I max out speed test sites at 150mbps or more.
Here's the network architecture, roughly:
inet --> Mikrotik Router --> Cisco 2970 --> Mikrotik Switch A --> SXT AC [ --- ] SXT AC --> Mikrotik Switch B
I've confirmed gigabit connection speed at the switches on both sides of the link. The bandwidth test was conducted between the two Mikrotik switches. Speed is crazy-fast plugged directly into Switch A in same port with same config as the SXT AC. Cable to both SXT AC devices tests good. Both Switch A and Switch B show gigabit connection to the SXT's.
I've tried 802.11 and the results are the same. Right now it's running NV2. Here's the wireless config:
set [ find default-name=wlan1 ] band=5ghz-onlyac channel-width=20/40/80mhz-eeeC country=\
"united states" disabled=no frequency=auto mode=ap-bridge nv2-cell-radius=10 \
nv2-preshared-key="REDACTED" nv2-security=enabled ssid=condolink tx-power=21 \
And the connected device stats (while running a test):
[admin@tjlinkap] /interface wireless registration-table> pr
# INTERFACE RADIO-NAME MAC-ADDRESS AP SIGNAL-STRENGTH TX-RATE UPTIME
0 wlan1 4C5E0C575115 4C:5E:0C:57:51:15 no -49dBm 130M... 1d50m57s
[admin@tjlinkap] /interface wireless registration-table> pr stats
0 interface=wlan1 radio-name="4C5E0C575115" mac-address=4C:5E:0C:57:51:15 ap=no wds=no bridge=yes
rx-rate="650Mbps-80MHz/2S/SGI" tx-rate="6Mbps" packets=397,203443 bytes=36220,307399137
frames=382,186677 frame-bytes=36280,307458376 uptime=2m8s last-activity=0ms
signal-strength=-50dBm signal-to-noise=52dB signal-strength-ch0=-54dBm
signal-strength-ch1=-51dBm tx-signal-strength-ch0=-52dBm tx-signal-strength-ch1=-56dBm
strength-at-rates=-50dBm@6Mbps 0ms,-50dBm@9Mbps 36s830ms,-50dBm@12Mbps 41s560ms,-50dBm@54Mbps
tx-signal-strength=-50dBm tx-ccq=6% rx-ccq=67% distance=1 routeros-version="6.33.3"
last-ip=10.95.1.5 encryption=aes-ccm group-encryption=aes-ccm
tx-rate-set="OFDM:6-54 BW:1x-4x SGI:1x-4x VHTMCS:SS1=0-9,SS2=0-9" tdma-timing-offset=2
tdma-tx-size=496 tdma-rx-size=4000 tdma-retx=4 tdma-winfull=0
Both devices are running RouterOS with wireless-cp, version 6.33.3, firmware 3.24:
[admin@tjlinkap] /system routerboard> pr
model: SXT G-5HPacD
[admin@linkshop] /system routerboard> pr
model: SXT G-5HPacD