I'm currently facing the following issue: On normal conditions, with just my laptop plugged in with ethernet, I have a download speed of 7.2 MBit/s and upload speed of 0,6 MBit/s. Now, I recently bought a RPi3 and configured on it a OpenVPN server, as well as a Media Plex server. Since then my download speed dropped to 1,7 MBit/s and upload at 0,4 MBit/s. Whenever I disconnect my Raspberry from the network I get full speed again, for this reason I know that the problem lies on the RPi. I assumed that the problem could be the OpenVPN or the Media Plex services, so I stopped those two services and I also flushed all my iptables rules, but nothing changed.
I then tried with nethogs and Wireshark to check if some process was using a lot of bandwidth, but unfortunately I got nothing, all was on normal values (nethogs showed 1-5Kbit/s Download and 0,5-1 Kbit/s upload).
So I now really don't know why this is happening and hope that maybe you could give me some help to figure it out. Ask for any config files or additional information and I'll post them.
Many Thanks in advance
Edit: Situation Update:
When I connect my raspberry pi 3 over ethernet, all my other devices (connected over ethernet or WiFi) are slowed down from 8 Mbit/s to 1,5 Mbit/s.
When I shutdown my raspberry, but leave the power supply attached to the raspberry, as well as the ethernet cable, then I get an improvement of 0,5 Mbit/s. Only when I disconnect my raspberry from power or detach the ethernet cable my bandwidth goes back to normal.