I think i've found the lagging cause. Probably it is leading to the main problem as well:
The grass object has Collisions enabled with Preview on Viewport. The density is very high (although most of items are being discarded by collisions). This is causing a significative performance hit each time Forest is rebuilt. Normally, should be necessary to rebuild it only if some parameter changes, but since there is a camera associated, the object is updated also when camera changes.
Even worse, by some reason the VRayPhysicalCamera is triggering an update notification when timeline moves. This is not normal, and it does not happen with the Standard Max camera. We will investigate the reason of this, and we will contact with ChaosGroup if it's necessary.
How to fix it:
- Turn off Camera->Auto assign to active view, and click Camera->Clear (in this scene, camera clipping is not necessary)
- Turn off Collisions->Preview on viewport
As information, i usually doesn't use collisions for this type of grass distributions. I prefer this workflow, which is much more efficient (specially for large areas):
- Set Distribution Map to "Full"
- Collisions off
- Transform->Translation on (10 or 20%)
- If needed, set Areas->Boundary Checking to Size
- Then adjust Distribution Map X/Y size to get the desired density.
Hopefully, the changes described above should solve all slowdown issues.