If you cancel EntityTransformEvent, this event runs indefinitely when killing a slime, never triggering SlimeSplitEvent or spawning smaller slimes, but it also does not despawn the slime that was killed, which causes TPS(CPU) lag and also leaves this in the world:

Code:

@EventHandler(priority = EventPriority.NORMAL)
public void onEntityTransform(EntityTransformEvent event) {     event.setCancelled(true);
}

Attached is the plugin jar if that helps.