When you have more vertices, the peak range point for the polygon count drops. The 3000 to 6000 range is where peak performance is when there are two triangles per vertex, the typical setup. For something like trees where you have triangle-shaped objects crisscrossing each other, the vertex count increases and the range would be more around 2000 to 4500 polygons for a 1:1 face:vertex ratio. By switching the vertex and polygon counts around, the one with more vertices will take almost exactly twice as long to render. Think of it as each vertex counting as 3 faces (I have yet to solve for the actual value). That is, if you have 100 vertices and 190 faces, the model would, in a sense, be the equivelent of 490 faces and 0 vertices. Vertices have a much greater impact on the frame rate than polygons do so you should use the fewest possible.

@FoxHound - That number is one decillion (1E33), not a zillion. No hard drive or memory can even store that many!