IP address conflict? How is your Pi network configured?

So I moved the Pi away from the router, but no improvement.
@Emerson: sorry for the dumb question but what do you mean by how my Pi is configured? Internal IP of the Pi is 192.168.2.103.

So is there another node on your network with same IP address?

No, all other devices use another IP address. The IP Range is set to 192.168.2.0/24.

I would say there is some electrical interference coming from some part of the pi since it is degrading your routers connection to the internet. If it only happens when the pi is connected by ethernet (and not just when the pi is turned on) then the interference could be making its way down the ethernet cable to the router.

mii-tool is deprecated and looks like it is giving you false information since the Pi only has a 10/100 ethernet adapter. You should use the ethtool instead. Are you only connecting the Pi only via ethernet? What happens when you connect via wireless?

I suspect the Pi has a hardware problem with its ethernet adapter. If you have changed cables and switched router ports then the only common item is the Pi.

Connected the power supply to the Pi and waited for it to start. Waited a minute and no sign of speed loss on my network. So the option mentioned by af7567 could be valid.

I was using the whole time, the Pi over ssh. Now I only attached my HDMI cable, my usb mouse and my usb keyboard (both wired) to the pi (so no internet connection), and something really strange happened.
My router lost the connection to the internet and reconnected itself as before, but this time I went from 8,1 MBit/s to 5,4 MBit/s on the whole network.

The ethernet cable was not connected and the wifi was off, so there was no internet connection on the Pi.

Update: if I connect the Pi via Wifi, my router does not reconnect, but the speed remain at 5,4 MBit/s instead of 8 MBit/s.

So I tried to do some HW troubleshooting, as far as it makes sense, what I did here.

1. Power supply, HDMI, USB mouse and keyboard (wired). WiFi disabled -> 5,4 Mbit/s
2. Power supply, HDMI, USB mouse and keyboard (wired). WiFi enabled -> 5,4 Mbit/s
3. Power supply, HDMI. WiFi Disabled -> 5,4 Mbit/s
4. Power supply, HDMI. WiFi enabled -> 5,4 Mbit/s
5. Only power supply. WiFi Disabled -> 7,1 Mbit/s
6. Only power supply. WiFi Enabled -> 7,1 Mbit/s

In every option the Ethernet cable was not attached.

So what I think is that the HDMI connector is causing this problem. But this does not explain why when the ethernet cable is attached, there is so much bandwidth loss, and it doesn't explain this 1 Mbit/s which is still missing to reach the normal bandwidth.

What are your more experienced opinions? Is there any other test I could do to figure out the solution?

Update: If I attach the Ethernet cable too, the speed drops again. The best situation I have is with the power supply attached, HDMI attached, WiFi disabled: 2,6 Mbit/s.
So its the opposite of before, where if I detached the HDMI cable the speed went similar to normal (7,2 Mbit/s).

Could this two things be correlated?

How far is the pi from any computer cabling? If it is RFI then covering the pi with a metal cover of some type might help. Their could be leakage from the cables.

Hardware bottlenecks are a possibility, and very difficult to solve.If everything's going into one big chip which may be passing video, memory access or whatever, and you're using all these things, expect trouble. The chip will only make one access at a time.

For what it's worth. My raspi 3 can speedtest 167Mbps over wifi when I run the test via proxying through the pi with another computer (via ssh -ND ...). I have an 802.11ac router. My internet speed is 225Mbps so it's safe for me to say I maxed out the speed of my Pi using it as a SOCKS proxy. My raspi power supply unit (PSU) is 5.25V/1000mA.

What kind of power supply do you have? Can you describe more about the specs of your raspi power supply? Model number and voltages/amps of your PSU would be helpful.

I've heard people having issues with power supplies that aren't designed well for this application.

1 members found this post helpful.

After a lot of troubleshooting, I finally found the cause of my problem.
My Raspberry Pi power supply, was causing some interferences with the router, for this reason the bandwidth was reduced.
Many thanks to all who helped me to solve this problem.

The pi 3 has higher power requirements than previous models - using an older power supply is specifically warned against.
FWIW I have had no issues using a pi 3 for a firewall/router in front of my network.