Points-cloud is probably the fastest mode, because the total count of drawn points is constant for each Forest object, regardless of the number of items.
I also suggest that you try disabling Camera clipping. Some scenes may benefit of using this feature (specially those with large areas of trees), but it introduces a hit performance because the Forest object must be rebuilt when camera moves. To disable it, turn off "Auto assign to camera view" and click the Clear button (in the Camera rollout).