Thank you for your reddit post. I tried to test my connection with that of the VPS via iperf3 like you did and surely enough I got almost the exact same results as you did.. Now I really don't want to reset the server or my computer either, so I will have to try out other things may could help me with this problem.. 
is that computer running plex player, windows or linux?
I use Windows 10 x64 2004 and the server runs Debian 10 x64. I even tested downloading a file with FTP from the server to my PC and even there I get the strange ~30Mb/s limit but per file and with Plex it is per stream.. I don't know whats wrong and can't find any solution what that weird invisible limit is? 
on win10, can you post the output of
netsh interface tcp show global
I hope it doesn't bother you that it is in German.. 
netsh interface tcp show global
Der aktive Status wird abgefragt...
Globale TCP-Parameter
Zustand der empfangsseitigen Skalierung : disabled
Autom. Abstimmungsgrad Empfangsfenster : disabled
Add-On "Überlastungssteuerungsanbieter" : default
ECN-Funktion : disabled
RFC 1323-Zeitstempel : disabled
RTO (anfänglich) : 1000
Zustand der Empfangssegmentzusammenfügung : enabled
Nicht-SACK-RTT-Widerstandsfähigkeit : disabled
Maximale SYN-Neuübertragungen : 4
Fast Open : enabled
Fast Open-Fallback : enabled
HyStart : enabled
Proportionale Ratenreduzierung : enabled
Schrittsteuerungsprofil : off
which one is autotuninglevel
Autom. Abstimmungsgrad Empfangsfenster : disabled
try this, sometimes a reboot is needed.
netsh int tcp set global autotuninglevel=normal
Okay I changed it and will reboot now, but what does it do, and how could this solve the problem? :o
take a read of this
as a test, you can run rclone mount on the windows computer and using vlc, play media.
also, easy to run plex server on win10
Oh my god! Thank you very very much that did it! I get the full ~100 Mbit/s that my local connection is capable in terms of download speed from the server! I didn't even need to test it locally with rclone mount because I directly got the full speed from the server!
happy to help a fellow rcloner on a happy new years day.
the crazy thing is, this post and the previous post, back to back posts, were both solved with that same netsh command
the only thing missing from this post is another like from @VBB....
1 Like
You rang?? 
How come this is not set to "normal" for so many users? Did you all mess with it, use some kind of optimizer or something? All my installations have this set to "normal" by default... 
Happy New Year, btw! 
...and try finding a word as long as Empfangssegmentzusammenfügung in the English dictionary! We Germans are a bunch of cunning linguists 
i assume you have read the original post, so i will delete it now...
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