Why does the router give out a lower speed than on the port?

Question

Computer networks

Good evening! There is a D-link dir-300A1 router software version 2.5.4 (Latest). Internet from Rostelecom. Speed ​​on a straight line, when connecting a cable to a network card 40-50 Mbps. When connected through a router, the speed does not rise more than 20 Mbps. Why can this be so?

Answer 1

Speed ​​on a straight line, when connecting a cable to a network card 40-50 Mbps. When connected through a router, the speed does not rise more than 20 Mbps.

What is "speed"? How do you measure it? Through speedtest.net and similar services? How do you get internet (is VPN or PPPoE used)?

Why can this be so?

Possible options (the list is incomplete):
1) The router cannot cope with the load. Check the utilization of its resources (CPU, memory). Bugs in the implementation of protocol support are also possible.
2) Suboptimal TCP operation (in case you use speedtest.net results). Check your MTU and TCP MSS (maximum-segment size) settings, if any. Check the udp datagram transmission speed with iperf for example.
3) Duplex inconsistency on any of the ports. Check the duplex settings and check for errors on the ports.
4) As a special case of option 1), the presence of built-in "security" mechanisms (firewalls, ids, etc.) that utilize resources. Try turning them off.
If you get internet through VPN/PPPoE, then I tend to go with the first option. It may be that the router is not capable of handling higher traffic.
In principle, the same options, only in a different wording:
1) maybe the router stopped functioning normally for some reason (poor power - unstable performance - dropping packets, or memory breakdown - checksum violation - dropping packets)
2) settings duplex on ports, especially on the WAN port of the router
3) problems with MTU should also not be ruled out
4) the provider or your neighbor on the switch started sending you spurious traffic, the router tries to process it, which leads to a decrease in useful performance.
5) the provider changed the PPPoE parameters, the router reacted with a decrease in performance due to bugs in the firmware.
The most complete picture will give a comparison of the characteristics of the connection through the router and directly (here you can study Internet traffic from the computer with different connection options and compare its characteristics), the study of the characteristics of the router (the number of dropped packets, errors on ports, resource utilization) and Internet traffic from its WAN port (number of retries, TCP parameters, presence of fragmentation).
Unfortunately, I cannot provide you with ready-made monosyllabic answers. "Home" routers seem to me rather inconvenient in diagnostics due to the lack of intelligible tools for this.

Answer 2

Maybe it's in the firmware. The total throughput is divided by the number of active ports.
Try other firmware.