Our server started being laggier than ever

Basically, when I joined last night, it was all perfectly fine. Then I decided to rejoin just an hour earlier and saw a lot of weird changes. One of the changes is that the entities such as item frames, armor stands, paintings, mobs, and players disappear despite only being 5 blocks away from them, they normally should be visible despite being far away. The lag seems to get worse as we cannot eat food and me using fireworks on my elytra seems to get out of control for despite doing various ways to slow down, I just wouldn't slow down which would cause me to crash and die. Please we need help on fixing our server for it suddenly has become so laggy.