I need help to understand why my server is lagging

Hi everyone, yesterday I was playing on a Craftoria server that I set up with a friend a few months ago, but it started running out of memory, so I decided to buy a paid server. In this case, I wanted to try thegamehosting.com. We tested it yesterday and it was working fine, but today we've been experiencing constant lag on the server and I don't know why. I ran a diagnostic with Spark and everything was fine, it wasn't overloaded, our internet isn't either, and I even installed mods and plugins to optimize the server, but it always happens that when one of us is on, the other gets incredible lag, and then it's fine, and then again we can't even move. Does anyone know why this is happening? I'm desperate.