What I'd be curious to hear about is how your servers handle simulating the game world. How many players are simulated per world and what are some of the tricks that you use to ensure the simulation doesn't fall behind, or if it does how do you make sure the clients don't notice?

I'm currently writing a (much much simpler agar.io-like) multiplayer game and finding it quite challenging to make the experience smooth, I haven't implemented any lag compensation yet though so maybe that's why but I wonder if that is the only problem. Do you or anyone else have any resources that could help me?