If I may ask, what device is this ISP modem, make, model, etc?
Any idea why vanilla config would have that issue? I always got fantastic lan performance, it just happens when hitting wan.RB2011 as a basic router can handle 890 Mbps of IPv4 TCP fasttracked traffic. Other configuration aspects can make it slower. Post config to be sure.
Thank you for trying! This is really frustrating.Sorry, without actually being at the network, can't offer more