Hey, I found this mod:
https://github.com/Nixuge/ServerlistBufferFixer
Do you think it's possible to port it to your mod?

Definitely not bandwith issue.

Hello! My mod was inspired by Nixuge's ServerlistBufferFixer, it does exactly the same thing. The implementation is a little different, as Minecrafts code has changed since 1.8.9.

All right, I found 2 mods, that at the moment greatly improve pinging:
https://modrinth.com/mod/bannedserverbypass
https://modrinth.com/mod/fast-ip-ping

I use them, that's what you can understand from my previous message, but after some time it gets slow again.

Great, you can download and use them! Unfortunately I won't include them in this mod as you can already use them as is and I don't want to steal their functionality.

What slows down exactly? Does that happen without my/all mods as well?

After some time or couple refreshes pings are infinite and you can't join any server, I think it happens in all scenarios.

Could this be a bandwidth issue? If your home internet is too slow, the router might stop additional traffic to wait for others to be sent. You can check here: https://speedtest.net