Too much ticks in a 500+ mods modpack.
Especially "zombie._SpecialZombieEntity.aiStep()"

Spark profiler:
https://spark.lucko.me/3mcY0WFLg5

I think the key information here is "500+ mods". Some of SM's mobs does have AI that is a bit more hefty than that of most vanilla mobs, but there is no way to tell just how many of the other hundreds of other mods play into this