It isn't a bug I don't think as it is in the game. But it is something to be fixed. The firework bow specifically being used by the crates at spawn cause extream lag. As well as the fireworks themselves cause damage in areas that have PVP turned off, allowing the killing of another player. The fireworks that kill as well are not associated with the player, so no death tag in chat saying X killed X with X, and no log (if I'm right) saying the player killed them, but that they were killed by a firework. If both of these can be fixed that would be nice, some possible fixes are to make it so fireworks entity cant spawn at spawn like any other entity. The other fix is to remove damage from fireworks when in towns.