BT HH5A - Slow Wifi throughput (5GHz) (not WAN)

Hi,

I'm running OpenWRT on a BT HH5A - currently version OpenWrt 19.07.2 r10947-65030d81f3
I read about the slow WAN-LAN speeds but I'm purely using it as an AP so that shouldn't be an issue.
With wired connections I'm at full gigabit speeds - hitting my internet connection max of 380 mbps (VM UK) and 950+ mbps locally across the LAN with iperf3. Internet connection is via a separate modem/router combination elsewhere on the LAN. No use of WAN on the BT HH5A

The problem is wifi speeds - specifically 5GHz wifi speeds. Are they known to be bad?
I'm getting a max of around 110-120 mbps - both over the internet and locally with iperf3. The UI is showing a wifi connection speed of 866 mbps and I'm literally sat next to the router.

Realise I'm not going to max 866 mpbs, but 110 mbps is particularly poor. I'd expect to be able to max my VM internet connection at 380 mbps and get close to 500 mbps wifi/LAN speeds. My previous Tenda AC10 was easily capable of this. Don't want to go back to it as I much prefer OpenWRT.

Any recommended firmware versions? (the modded 18.06? but that's a WAN/LAN fix)
Any recommended 5GHZ settings?
Current settings
Mode AC
Channel 100
Width 80 MHz
WMM Mode enabled
Max transmit 20 dBm
Otherwise defaults.

Yes is simple answer to poor LAN to wifi speeds. Don't expect more than 170 Mbps in multithreaded speed tests which is still less than theoretical 433 Mbps max (433 transmit + 433 receive = 866 Mbps). See section 7.12 of installation guide.
https://openwrt.ebilan.co.uk/viewtopic.php?f=7&t=266

imho, if you want faster LAN to wifi speeds, always use a dedicated WAP running OEM firmware.

fwiw, Amazon UK have an offer on RT-AC57U AC1200 router/AP/repeater. It supports OpenWrt too. https://forum.openwrt.org/t/best-openwrt-router-for-50/38795/80
(Also, search for 'rt-ac57u' at hotukdeals)

Same problem here. Have you tried the modded v18.06 openwrt on it?