I'm afraid this is a problem exclusively of Max 2014. Autodesk (or NVidia) has introduced significant changes in Mental Ray, probably related with the new unified sampling system. For some reason these changes affect to the render performance on heavy scenes.
Forest works exactly in the same way that previous versions, and it has not control over the rendering process. The current release is completely compatible with Max 2014, and there are not changes that we can do.
Anyway, i suggest you upgrade to Max 2014 SP1 and repeat your tests. I just rendered a sample scene (Lake with Pier 1.0
) in Max 2013 and 2014, and got exactly the same render times (4'04''). We'll try using heavier scenes.