Diagnosis
The router's hardware can handle gigabit, so a major slowdown usually points to a bad QoS (Quality of Service) setting, a wonky cable, or the built-in speed test influencing things.
How to Fix It
First, disable QoS. Go to Adaptive QoS > Bandwidth Monitor in the web interface. If 'Bandwidth Limiter' or 'Traditional QoS' is enabled, turn it off. Also, go to the 'Speed Test' tab under Adaptive QoS and make sure the 'Automatic QoS' option is disabled. Reboot the router after making these changes.
If That Didn't Work
Test with a different, known-good Cat5e or Cat6 Ethernet cable between your modem and the router's yellow WAN port. Also, try connecting a computer directly to one of the LAN ports with another good cable to rule out your modem or ISP as the bottleneck.
Heads Up
The built-in Traffic Analyzer and AiProtection features are lightweight, but for the most accurate raw speed test, temporarily disable them both (under Administration > Privacy) and run a test from a wired PC using speedtest.net or your ISP's tool.