I've had the problem for a long time and it's getting really annoying.
Got a pretty fast internet connection, speedtest says 9ms. Can use the fast internet anywhere, e.g. High-resolution videos on YT etc.
But as soon as I'm in a Minecraft server, no matter which server, my internet there's extremely slow and it is almost unplayable. It's a shame, I often wanted to play with friends. Otherwise the internet works fine, also in other games or devices.
Didn't find anything about it on the internet and hope that you can help me somehow.
Do the following in the command line
tracert IP-des_server [Enter]
This shows you the latency at every jump point between you and the server.
If it's the last entry that has high latency, it is probably the server.
But it can also be that something in between is the eye of a needle.