When a TNT riding a firework explodes the framerate drops to 0. I have reproduced this bug every attempt in many different world with slightly different spawners. The framerate does not suffer at every explosion, but it will always be affected within a few detonations. The only solution has been doing into MCEdit and deleting the TNT Firework entity. This removes the lag until it occurs again. I think it has something to do with a firework entity being killed by TNT.