Yes, it is a strange issue. I think the problem would be in Mental Ray itself, because once Forest converts the items to render instances (just after the translation), the plugin practically doesn't participate in the rendering process (except if Forest Material or Forest Color are used).
Probably the problem happens only with Forest because it generates thousands of render instances, stressing the render engine. Usually a typical Max scene contains much less items.
I would like to confirm it. Please, try this:
- Create a Forest test scene with roughly 50.000 items (you can see the number of items from General->Forest Stats).
- Use Forest Tools (Utilities Panel) to convert Forest to stand-alone objects.
- Remove all the Forest objects from the scene.
- Compare the render time between the different render nodes.
If the problem is reproducible or there are significative differences, the problem is not in Forest itself. In this case, should be needed to contact with Autodesk. If not, we will investigate further.