Precisely! Actors (such as NPCs, creatures, thralls and pets) are the most resource intensive entities in any game like this: they increase the processing load both on the server and the client. On the client, they have to be animated and synced with the server. On the server, their AI has to be simulated and, depending on how strictly server-authoritative the game is, additional simulation has to be executed, such as skeletal animation.
After that come the placeables – like torches, banners, crafting stations, and such – which increase the processing load on the client (animations, lighting, shadows, and such). Crafting stations also incur a small penalty on the server performance, due to the “catch-up logic” that has to be executed on the server when a player “wakes them up”.
The least resource intensive entities are the building pieces. They are static and their resource consumption boils down to memory usage, the network I/O when they’re loaded in from the server, and the rendering.
So yeah, too many thralls in the same place is a problem on PVP, PVE and PVE-C alike. Or any other server, really.
What I’m wondering about is something they said on the Dev Stream. Alex said that “once you hit 650 followers the game performs very differently”. I’m very curious about whether that’s 650 in one place (e.g. within the rendering distance) or for the whole game. If it’s the former, the cap should help, although I still insist that a density cap would be better solution. If it’s the latter, then the cap is of dubious worth, because I’m pretty sure that a decently populated server will hit 650 followers easily.