What eventually helped: swap “LAN” ↔ “WAN” ports. My internal LAN is gigabit, so both eth0 and eth1 in the router work at gigabit speed, but it still helped.
In case someone will want to know how to replace LAN with WAN: someone explained how to do that in the GL-MT3000: GL MT-2500 LAN and WAN port swap and switch option missing - #3 by K3rn3l_Ku5h