I've just opened an aternos server that I can play with my friends, but the server server lags extremely, there are no mods on it and 16.5 is the latest version
Well that's what aternos servers are known for, just buy a server for a euro or something
Alternos is really bad the servers lag very badly I would play hamachi if you didn't want to spend any money
This is normal with Aternos. There's very little RAM available.
Hamachi is very antique and bad and everyone who wants to play needs it. I would recommend Ngrok.