Install the Merlin Firmware build for your Router. I have a 68u and found that enabling Jumbo Packets, disable AI protection, etc allows me to get the max speed for my connection. It is a lot of trial and error to get it stable. I had to go in and disable Power mgmt, energy efficient ethernet, green ethernet, selective suspend, everything wake up lan.

Increased Interrupt Moderation to high, Jumbo Packets is 4k, Speed & Duplex is 1000. I am using a USB 3.0 to Ethernet and get 980 with the Okla Speedtest app for Windows to Comcast Chicago.

Asuswrt-Merlin - Browse Files at SourceForge.net
Snbforums .com has a dedicated forum group for Asus Merlin.