Huge render speed difference. Why?

[deadakos]

I'm asking your help in understanding the following situation, or to find a solution to it.
I have realized during my last personal project, that the rendering speed decreased dramatically after adding a tree model to the scene.
I used a Plant Factory tree exported to obj, and rendered with PT kernel in standalone.
Today I made a short test with a quite crowded tree model from Evermotion, and with this Plant Factory tree model.
Here are the details of the models:
Plant Factory model size in vram: 1001 mb (1744424 tri)
Evermotion model size in vram: 1076 mb (2115840 tri)
I set up a simple hdri environment (above written vram usage is calculated with the hdri image loaded).
Rendering time for the PF model was 8:25 minutes, and 28 seconds for the EM one. EM rendered 17 times faster.
Why?
How can I optimize the very nice model of PF to reach the same rendering speed?

[uncia]

I believe that Plant Factory tree has alpha on leaves that is why it is so slow

[deadakos]

You mean alpha in the opacity chanel to create the shape of the leaves?
Both model uses square shaped bilboards for the leaves.

[deadakos]

Any idea?

[deadakos]

I figured out. The PF model had much more detailed leaves, and the LOD needed to be decrease to reach the same rendering speed.

[Seekerfinder]

Hi Deadakos,
So you changed the export settings in PF?
Is the compromise in quality dramatic?
Is PF flexible in this way? Export options etc
Cheers,
Seeker

[deadakos]

No, I just changed the level of detail of the leaves, and then exported the tree as obj.
I use Plant Factory Exporter, not the full version (it's free). The bought tree models have the option to change the LOD of the branches, leaves, flowers/catkins, etc. separately. I left everything else unchanged, just decreased the LOD of the leaves, and it helped a lot.
I experienced that decreasing the LOD of the branches will destroy the overall outlook of the tree, but changing only the leaves, the tree remains almost so beautiful as you can see in the PF viewer.
I can't tell you how flexible the PF is in exporting the models, but PFE has just a few options. Basically, you don't have any influence on the export quality.