I just implemented a shader instancing thanks to the example in the snipet section. I have now 450 fps in the worst case (had 70 in the same case without instancing). But but my instancing is not using the light in my scene and all the object in my scene use the same object (a sphere). I'll come back with more benchmark when I have different objet to instanciate.
Thanks anyway for all your support

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Edit : Finally the VBO started working when I reached 500 polys on each of my scene nodes and it is now a little faster than the shader instancing :p.