EDIT 3: Now the waiting "copying transformation to the device" is pretty much gone! (it initialized a lot faster now)
(it seems there is a bug making particle instances transfer slower when they are scaled, so apply the transformation to the mesh (ctrl+a) workaround the problem )
EDIT 2: Blend File updated and optimized to render TWICE as fast and looks right!
(not taking into account the BVH building time)
The BVH building time is considerable for this file in a single frame render with a powerful GPU, so it is not really good file for BENCHMARK.
But for animation with BVH caching that problem goes away!
EDIT: Blend File at the bottom of the post
I wanted to test Cycles performing landscapes, so i setup a quick scene with 54K instances of two tree meshes each one aprox 200K and 150K polygons and four texture maps about 512^2pixels, the blender memory indicator displayed about 200MB and the process window showed about 500MB ram so there is a bunch of memory left to put a lot more. Nice!
And not that bad in time, in my old CPU Phenom-4 cores they took about 40 minutes in Full HD and 100 samples, but at 40 samples they look quite acceptable.
A little glow and fog and color correction in the compositor.
EDIT: Guys thank you very much for the nice responses, i also want to share the credit with you all because if this community of developers and artist wasn't here, my means to do it would be severe reduced
Here is the blend file for whatever you need: i uploaded in Blendswap and Dropbox:
UPDATED .blend: (2.62 with BMESH is required otherwise the background is shifted)
I would really want to see tested it in your machines, specially in any GPU, would be nice to see the timing!
EDIT 2: Optimized to render twice as fast, and fixed compo nodes making look it bad!
-Thansk to @michalis to realize that i forgot to delete the mix shaders i didn't use in the leaves.
So if you are going to render leaves with translucent shader and if they will be seen far away don't mask them, or mix with any other shader, maintain it simple, and try to set the max transmission setting to 1, the change will not be visually appreciable for this kind of scene and you will get much more speed up!
- Also i didn't know muted shader actually will BE EVALUATED in the compositor after the F12 render, if you didn't realize that, you were getting a very different result from the original screenshot!
Lesson learned, after you finish something, delete everything that won't be used in the final release!!