Server Cant Keep Up

I have a vanilla multiplayer server with my friends and after a little bit the server lags a lot. We are playing on 1.14.2. It gets really laggy and the server crashed when we went into the nether. It starts lagging back and freezing when there is only 2 people on the server. There is a total of 4 people on when we are all playing. I constantly get a message in the console saying "Cant Keep Up! Is The Server Overloaded? Running [blank]ms or [blank] ticks behind. I dont know what to do and we cant play because the server lags too often.