Hello, Álvaro.
To be honest I'm not sure... Embree is intended for object intersections and I'm not sure if/how can it be used to store photons or if it would improve anything. I will keep this in mind but I will leave it for a later stage.
As you already requested, it's still too soon (I've made quite some significant changes to the code and the Blender exporter today for example), but just as a "rough" comparison test I have attached the render logs for a modified teddy scene with both v3.3.0 and the development v4. You can see all details in the log.
This render is done with photon mapping and contains several thousand instances to create the "fur". One significant difference in the v4 is that we no longer need to create the old slow/single threaded KdTree!

That saves a lot of time. For example in the v3 version, the KdTree generation takes 1min 30s, while in the v4 version a BVH is generated which takes almost no time to generate

This comparison was done with the same scene, which also contains some Blender subdivision modifiers. I could have done it better for v4 by removing the Blender subdivision modifiers and using the new direct subdivision meshes, but I wanted a "same scene comparison" here.
I hope you find it informative, but please remember this is still too soon and things can change a lot still!
The lighting in the teddy bear in v4 is somewhat different for some reason, although I'm not sure whether this is more correct in the v3 render or in the v4 render? (v4 looks better to me but I could be wrong)