Not speaking for the other people (although I'm happy that many other people are commenting about the issue), I have a pretty beefy setup so maybe it's my girlfriend's PC, because that shit need work feel me. It's an all-in-one.
Once we have NPCs, we can have proper succession games, where when you die you can take over as another NPC in your camp.
This allows for waaaay more endgame content, depending on how well the NPC AI is coded. There are other games where large numbers of NPCs can be managed in colony-mode, although that may be beyond the scope of Zomboid. I'm thinking Dwarf Fortress and Rimworld here, but this creates a lot of extra layers of NPC behavior and management beyond what would be required for "collect survivors into a camp", which that one mod does for Zomboid.
My hope is TIS would go that route, of course, and implement at least a skeletal structure for more colony management, effectively creating a second game within the first. Then you get the joy of building up your own colony, plus diplomacy/fights with other colonies, mergers, attracting immigrants and refugee groups, and generally rebuilding society from scratch, all while fending off zombie hordes. Also there's intra-colony factional struggles to manage. Say you die, you take over as your next-in-command, but maybe they don't have the same influence in the colony and end up being subjugated to an AI-led colony, where the RNG personal characteristics of a new leader take it in a whole new direction, and you have to work to build up your own faction and influence to take over again. You can go so many ways with this it's crazy. Get all CKII up in it.
I'd call it Rimzoid, but that's why I'm not in advertising.
They could always release it as DLC/expansion, I'd buy the s--- out of that to support them for the added features. My hope would be that TIS would split off into "next game" staff and "add lots of new AI/NPC stuff via DLC to Zomboid" staff. Steady income stream for a couple coders. I mean, we, the fanbase, are pretty used to waiting a long time for content now, so no rush. I'll wait a few years and buy a "colony simulator" expansion to Zomboid no probs.
One thing that I have noticed myself is that having more zeds nearby increases load on the server.
Example: this graph is a load on the host for 1 player roaming around a base with some zeds, just your normal play on foot up till halfway. Then 2 helicopters happened, and player hides in a base with ~150 zeds going for him.
As you can see, that's about x2 load you get for additional 130-150 zeds at once nearby.
You can try to up your server hardware (mostly CPU) several times and see if that helps it.