Hello,
I have some strange issue.
currently I'm using firmware version 4.8.5 I readed that the negotiation bug has been fixed.
I also are connected to the ethernet ports when testing.
when I go to speedtest, speedtest errors Download Test Error this is often suggests there is packet loss or MTU problems, when it works it can take serious long time to find a server.
when I visit speed.cloudflare.com I see an considered low speed. 80mb/s, but the upload speed is fine 300mb/s.
iperf3 shows good speed, waveform scored A+ with the correct speeds above 1gbps, with bidir also works fine.
but something isn't right that speedtest drops the test, I have disabled any scanning software and without any extra vpn overhead, the only changes I made was:
insert my vpn config and test it, then disable it.
enable ipv6 (nat mode)
remove
/lan/from both wgclient as that from lan in luci's dhcp settings.
I have tried setting all my ports with this command:
ethtool -s eth0 autoneg off 1000 duplex full; ethtool -s eth1 autoneg off 1000 duplex full but I had to restart, my ipconfig /release && ipconfig /renew did not want a ip.
are there any known regressions?, I might have to reconsider my test location in my topology putting my test location closer to the main router, the Brume3 however did not have shown this problem which has been previously connected to it.
edit:
I noticed a bit of dmesg log:
[ 33.937323] mt7993 0000:01:00.0: connac_if_config_bss(): fw_idx 5, omac_idx 12, wmm_idx 0, band_idx 1
[ 33.953476] mt7993 0000:01:00.0: mtk_hwctrl_sta_alloc(G): mld_sta_idx=65535, wcid=517, hw_sta=00000000c1307468
[ 33.989786] mt7993 0000:01:00.0: connac_if_mld_add_link(): grp 20, mat 37, mldmat 37, remap 255
[ 33.998595] wlan_config_set_t2lm_nego_support: cfg t2lm_nego_support = 0
[ 34.005336] wlan_config_set_emlsr_mr: cfg emlsr_mr = 0
[ 34.010508] wlan_config_set_trans_to: cfg trans_to = 0
[ 34.015676] wlan_config_set_eml_omn_en: cfg EML OMN enable = 0
[ 34.044048] u1CCKTxStream = 4
[ 34.047181] set wrong parameters
[ 34.116403] vow_mbss_grp_band_map not support for VOW_GEN:4
[ 34.136009] UniEventEDCCAThresholdProc: fginit = 1, EDCCA Threshold=187, 190, 193, 200
[ 34.148520] mtk_ppe_dev_register_hook : ineterface rai15 register (6)
[ 35.743163] mtk_soc_eth 15100000.ethernet eth0: Link is Up - 2.5Gbps/Full - flow control off
[ 35.751667] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
[ 35.758451] IPv6: ADDRCONF(NETDEV_CHANGE): eth0.1: link becomes ready
[ 36.219214] [add_dev_config 319]add node success. iface:wan, dev:eth0.1, ifindex:8
[ 37.536559] -->update_qos_map(), update qos map config.
[ 38.299316] sh (16524): drop_caches: 3
[ 39.215590] no net device found for lan0 or apclii0
[ 39.220575] no net device found for lan1 or apclii0
[ 39.225501] no net device found for lan2 or apclii0
[ 39.230410] no net device found for lan3 or apclii0
[ 100.161825] TCP: br-lan: Driver has suspect GRO implementation, TCP performance may be compromised.
[ 101.418716] mt7993 0000:01:00.0: connac_if_mld_add_link(): grp 1, mat 36, mldmat 0, remap 0
[ 101.427078] wlan_config_set_t2lm_nego_support: cfg t2lm_nego_support = 1
[ 101.433781] wlan_config_set_emlsr_mr: cfg emlsr_mr = 0
[ 101.438913] wlan_config_set_trans_to: cfg trans_to = 0
[ 101.444042] wlan_config_set_eml_omn_en: cfg EML OMN enable = 0
[ 101.963570] UniEventEDCCAThresholdProc: fginit = 1, EDCCA Threshold=187, 190, 193, 200
[ 201.770901] mapfilter:drop IP addr timeout! stop dropping IP addr.
[ 605.036511] mt7993 0000:01:00.0: mtk_hwctrl_sta_alloc(N): mld_sta_idx=65535, wcid=1, hw_sta=00000000aabe017a
[ 605.062242] 7993@ERROR.MLME,rai2,ap_cmm_peer_assoc_req_action() 3311: 26:0c:b6:78:36:c6, DSCPPolicyEnable=1
[ 605.099781] 7993@ERROR.RA,rai2,UniCmdSetRecSecPnInfo() 6006: wdev is null!
[ 605.106718] 7993@ERROR.RA,rai2,UniCmdSetRecSecPnInfo() 6006: wdev is null!
when I dissect this a little I see:
[ 100.161825] TCP: br-lan: Driver has suspect GRO implementation, TCP performance may be compromised.
and I see:
[ 34.047181] set wrong parameters which seems to be associated with QoS map, but its not really clear.
I can check what happens if I disable GRO 
edit: nope with command ethtool -K eth0 gro off gso off tso off on both ports no success.
edit:
I also tested some more I can confirm when my pc is connected to lan1, and the router is set on repeater mode 160hz there is no issue I get full throughput as expected, so the problem seem to be isolated to wan port or the communication between wan port and lan port.
edit:
I forced my pc to use 1gbps negotiation and speedtest works with zero problems, something goes wrong with negotiations 
I think it might be wise to post my network card because of (realtek stuff the buffer is 1024 maybe this says something?).
it is an Realtek Gaming 2.5GbE familly controller from the motherboard Asus TuF (Gaming B650-plus Wifi) revision 1.0.
edit:
inspected one of my smaller temporary patch cables these passthrough connectors always have some problems for me, saw rubber not fine terminated so decided to add classic rj45 jackets after my negotiation rate change it kept normal even when changing back, so I think the device is more vulnerable to bad cables even though it worked without problems, I gonna observe this a little longer 
Final edit:
The issue was on my side so there is no issue, I'm super happy with this travel router, same size as beryl ax but monsterious in speeds thanks to the cpu.