There are two changes that may improve the viewport performance:
a) Disable Camera clipping.
Camera clipping is useful for large extensions of terrain, but forces to rebuild and redraw the Forest object more times (each time that view changes). Also, this unables to cache efficiently the geometry in the video card.
Depending of the scene, in some cases you will get better performance enabling it, and others not. In the latest Forest versions, the plugin suggest to turn on this option only when using large areas. So, just try both modes and compare. To disable it, turn off "Auto assign to active view" and click "Clear" in the Camera rollout.
The Points-cloud mode is very nice, but depending of the video card and the number of visible objects, it would have a great impact on the viewport performance. Sometimes may be useful to try other modes and compare (as Display->Proxy->Pyramid).
Some tips about Points-cloud:
- Quadro cards are much better than gaming cards, when using wireframe and points-cloud modes.
- The total number of "points" is proportional to the number of visible Forest objects. In Forest 4 we have included a new parameter, to modify the global density of dots easily.
- There is a performance issue in Max 2013 (using Nitrous) with this mode. Autodesk is aware of the problem and they should fix it in next patches.
I hope that helps.