Manually added the 2.6.0.1 firmware and build from source, same issue.
I've noticed that setting the distance/ack to 1 increase speed from 50Mbit to 120Mbit at 30dB
And setting it to 2000 with 10dB brings speed down to 20Mbit from 400Mbit, seems to have a big effect.
Also the closer the Slate is at 10dB the slower the speed. I've also noticed that the TX/RX rate keep jumping from 6Mbit to 916Mbit gradually in a loop at 30dB, while it loops between 514Mbit to 916Mbit at 10dB. Hopefully someone with a Xiaomi AX1800 can chime in since they use the same chip (?).
Some relevant info with a similar firmware issue: OpenWrt support for Xiaomi AX9000 - #335 by dchard