Mob Lag

Hi. I have a problem with my server. I have no lag in terms of pickaxing or opening chests, but I have a lot of lag with mobs. Do you know something to solve this, like a mod to limitate entities or a command? Thank you so much for the server, I appreciate your hard work

I did it but they respawn for example in dungeons and it's the same again. I'm searching something like a mod. Thank you