I’ve been using this router since 2022. Before that, my ISP provided 100 Mbps, and I had no issues. Now that I’ve set up GPON, I’ve noticed that the connection speed drops significantly when using the router compared to a direct connection.

Test directly from the terminal:

Terminal - Archer C80 - PC:

The tests were conducted using the Speedtest desktop application for Windows, connecting to the same server at one-minute intervals. Both patch cords are CAT 5E.
IPTV and QoS are disabled, NAT acceleration is enabled. I even tried disabling SPI for testing - the download speed just won't go any higher.

I ran some synthetic tests using iperf:
1. WAN to LAN
The PC (server) is connected to the router's WAN port (simulating an ISP with a static IP), and the laptop (client) is connected to the router's LAN port

2. LAN to LAN
The PC (server) and laptop (client) are connected to the router's LAN ports

Direct routing over the LAN is 937 Mbps, while routing between the WAN and LAN (which should utilize the router's NAT) is 925 Mbps. The difference with NAT is only 12 Mbps.
 

I don't understand anything at all. Why does the download speed drop so much when using a router in real-world conditions (when running a Speedtest to the same server) compared to a direct connection to the terminal?

P.S. The ISP uses a standard dynamic IP address, not PPPoE