Is there any mod out there for creating lag on purpose? I need to cause server-client latency, but I'm sure tick and framerate lag would be useful for some people.

Just spawn 10k entities that dont do anything and the game will lag horrible

Question is rather famous "why?".

 

Per-player lag? Client or server side (meaning: which one can you access)? Global lag?

Mainly to test how my mod will deal with it. It does player movement and I want to know if the client will properly simulate the motion client side with server latency, and if the server will complain if there is latency and it thinks the player is flying.

 

On a related note. If I have motion simulated on the server as well as the client and the player is mid-air, will the server complain that the player is flying?

On the client or server side, set something up that runs each world tick, use a random chance to execute, then inside the execute, put a sleep command.  you will have to play with the random chances and sleep to get it right, but should work.