Archer C6 WAN PPPOE Speed Drop

Hello. I flashed OpenWrt SNAPSHOT r10127+231-3209f5ae3d with default .config on my router TP-Link Archer C6. The connection to Internet is PPPOE.
After changing the speed limit from the carrier from 60 Mb/s to 400 Mb/s I noticed that speed is not even a half of declared. After testing it is clear that the carrier really provides 400 Mb/s (direct connection to PC) and LAN ports are really 1 GB/s (transfer of huge files between wired PCs in the LAN). The only choke point is the WAN port on the router. The speed is not even consistent. On some test it is 200 Mb/s and at other times it can drop to 50 Mb/s. The upload speed usually +100 Mb/s of download.
Any ideas what can be done in this case?
P.S.: it is not possible to flash the stock firmware in my environment because TFTP method (which I used to flash OpenWRT) doesn't help
P.P.S: I tried to use a fresher build (SNAPSHOT r11xxx) but it didn't help

Please enable "Software flow offloading" in Network -> Firewall

Tried enabling "Software flow offloading", then "Hardware flow offloading" and disabling "SYN-flood protection". No changes on the first and even worse with the second.

Flashing the stock firmware bricks it. Flashing manually stripped stock firmware bricks it. Changing WAN port does nothing. Speedtest starts with high speed (over 400 Mb/s) than after 3-5 seconds drops to a half of it.