MT7615E Extremely slow speed

Xiaomi Mi Router 3 Pro 5/2.4 GHz

Speedtest on the router:

Download: 582.10 Mbit/s                                    
Upload: 491.91 Mbit/s  

Speedtest on a client:

Download: 30.43 Mbit/s
Upload: 26.16 Mbit/s

/etc/config/wireless

config wifi-device 'radio0'
	option type 'mac80211'
	option path '1e140000.pcie/pci0000:00/0000:00:00.0/0000:01:00.0'
	option channel 'auto'
	option band '2g'
	option htmode 'HT40'
	option cell_density '0'

config wifi-iface 'default_radio0'
	option device 'radio0'
	option network 'lan'
	option mode 'ap'
	option ssid 'OpenWrt_Legacy'
	option encryption 'psk2'
	option skip_inactivity_poll '1'
	option disassoc_low_ack '0'
	option key 'xxx'

config wifi-device 'radio1'
	option type 'mac80211'
	option path '1e140000.pcie/pci0000:00/0000:00:01.0/0000:02:00.0'
	option channel 'auto'
	option band '5g'
	option htmode 'VHT80'
	option cell_density '0'

config wifi-iface 'default_radio1'
	option device 'radio1'
	option network 'lan'
	option mode 'ap'
	option ssid 'OpenWrt_New'
	option encryption 'sae-mixed'
	option skip_inactivity_poll '1'
	option disassoc_low_ack '0'
	option key 'xxx'
	option ieee80211w '1'

Start with the supplied /etc/config/wireless settings and reproduce the router/client throughput gap on both bands. Trace the MT7615E/mac80211 driver path relevant to the configured HT40 and VHT80 modes; done means identifying the cause and verifying improved client speed against the reported baseline.