@Pasimfic_Wiki I have a lengthy answer here for you. Let us know if you have any more questions!
One thing I will preface about how we make worlds, we have to be very aware of FPS on lower spec systems, so a lot of our "guidelines" are driven by this.
This is from Rebecca, our WB team lead:
1) We don’t have any way to group objects together other than trees, so yes, it is better to think in terms of chunks when it comes to tree placement, following the guideline to try not to use more than four tree/plant types per chunk. When trees are grouped, each group counts as an object. If one tree type is across a chunk line and there are no other trees of that type within the chunk, the lone tree will be counted as a separate object. By moving it into the other chunk, it then becomes part of the group and will reduce the tree object count by one. It is really hard not to exceed four types per chunk, but with this goal in mind, it helps keep the number reasonable!
2) The overall general rule is that the more world object types per chunk, the more you are asking from performance. There are no magic numbers, but again, a warning to try and keep the variety of world objects used within a chunk to a minimum.
3) Lot objects mostly disappear with the LOD so the variety of BuildBuy objects on lots is a bit of a non-issue (but don’t go crazy). However, total object numbers per lot and their complexity, both script-wise and vert/drawcall-wise will make a difference! For example, if you use too many fences with a high vert count on a lot, the frame rate will drop.
4) Total object count in the entire world, both on and off lot, will affect performance.
5) Using layers in CAW: I’ve never tested whether trees will group across layers. I do know for sure that they group per chunk. I’ve never seen any benefit to creating layers per chunk. Since there is no advantage, really, it makes more sense to put things on layers according to one’s own organizational preferences. They are merely a tool to help with organization and have nothing to do with performance.
Hope this helps!