Wipe the router with a factory reset and do a minimal configuration. Only set the passwords and configure the Wifi ssid and passwords.
Many routers to get high speed have a hardware offload feature that move the NAT function off the main cpu chip. If the traffic goes via the CPU many times you get rates similar to what you are seeing. Many times it is lower but you have a fairly fast cpu in that router.
By default the hardware assist is activated on most routers. The problem is to use almost any feature on the router other than simple nat the traffic must pass through the CPU. Things like firewall and QoS are common things but even simple utilization reports need the cpu to be able to see the traffic.
Pretty much you are...