I've been having the same problem, mostly when the bride and groom are around.
I have to try to get away from them, and the further away I get, the better it gets until it's mostly normal FPS again.

Tomorrow I'll try disabling the Wedding group and see how that goes.

It was a little choppy when 3 NPCs in track suits attacked me with dumbells, but at least with them I was able to still play through and kill them ok.

When the bride and groom are around, I can count at least 5 and usually more actual seconds between frames...

I have been seeing this error streaming in the console log. Thousands of errors.

WARN : General f:3340, t:1759693392293> IsoGameCharacter.doDeferredMovement > WalkTowardState but path2 != null
WARN : General f:3346, t:1759693392624> IsoGameCharacter.doDeferredMovement > WalkTowardState but path2 != null
WARN : General f:3348, t:1759693392674> IsoGameCharacter.doDeferredMovement > WalkTowardState but path2 != null
WARN : General f:3351, t:1759693392765> IsoGameCharacter.doDeferredMovement > WalkTowardState but path2 != null
WARN : General f:3354, t:1759693392796> IsoGameCharacter.doDeferredMovement > WalkTowardState but path2 != null
WARN : General f:3356, t:1759693392807> IsoGameCharacter.doDeferredMovement > WalkTowardState but path2 != null
WARN : General f:3361, t:1759693392872> IsoGameCharacter.doDeferredMovement > WalkTowardState but path2 != null
WARN : General f:3362, t:1759693392883> IsoGameCharacter.doDeferredMovement > WalkTowardState but path2 != null
WARN : General f:3367, t:1759693392980> IsoGameCharacter.doDeferredMovement >

I had those warnings as well, but nothing really pointed to the actual problem unfortunately that I could tell.

I also figured the only other mod I had on that would probably effect the Zombies and therefore the NPC's as well would be "A Advanced Zombie Mod", so I just disabled it and did a Soft Reset and now it's working smooth like it should!

I'm guessing the 2 mods were trying to control them at the same time somehow or maybe the A Advanced Zombie Mod was constantly hammering the NPC's trying to instruct them but then kept losing their focus due to the Bandits mod or something...

Anyway, this is what just fixed my very serious lag problems where it took me about half an hour to get my character to walk about 10 feet to get back in the car and to drive away from the Wedding couple, which once I got a mile or so down the road it as smooth enough to log out safely without worry about not being able to log back in after.

They are still there at my base and I have no idea how to interact with them other than hitting G to mark a couple of squares for them to guard apparently, but they are super smooth now, AND I also got attacked by another hostile group of NPCs which the Wedding couple helped kill, and that was also all smooth like it should be!!!

So I guess you should check to see if you are using that "A Advanced Zombie Mod" or any other mod that you figure might be interacting with zombies in any ways other than their clothing, like just behaviour wise...

I have Authentic Z on and it's fine to use since it only adds clothing and or equipment to the zombies, and does nothing to the zombies behaviour and or action settings...

I also have the zombie death animations mod on as well, which is also fine to use, as it only effects the way they die.

I hope this helps you guys out?!?!?!