Any event that involves spawning ghast (Ghast Survival) causes lag.
If I am not mistaken, doing an action that causes lag for an extended time (like say 10mins or more) is technically illegal since it disrupts the server.
It would be different if everyone on the server was in the event, but that rarely happens. The lag caused by events is unfair to those not in the event.
Think of it this way. Lets say I am a new player joining the server for the first time and the server is lagging because of an event. Why would I, the new player who is now lagging, want to stay on the server?