When 1.13 came out for SS, there were a lot of mobs just grouped up together when exploring new chunks, if that's what you mean by high spawn rates, that's just minecraft being minecraft. i think...
There's high amounts of mobs spawning in old chunks as well. My base is literally infested with pigs and cows right now, along with other mobs. Regardless, I trust Shaz knows what's going on and will handle it at some point.
We also expanded the border and until those chunks get loaded there will be lag. Shaz has been informed about the mobs. Please follow the 30 mobs per chunk limit and please so not allow a ton of mobs to spawn at grinders before killing them. Every little bit helps.