Many thanks for those advices!
I am new to org-mode but, is there a reason why #+BEGIN_proof #+END_proof and other org-latex-special-block are treated as block ?
I mean; those #+... aim, as far as I understand, to give tips to org-export for prettier exports but nothing else (between those #+... you still need to write in org). I highlight that org-latex-special-block have nothing to do (in my point of view) with #+BEGIN_latex / #+END_latex that allow a user to insert some LaTeX within the document (e.g. Tikz pictures).
Therefore, it might a great idea to take #+BEGIN_proof & co into account only when it comes to export the document. The proofs/definitions/theorems will no longer be considered as block, and fast fontification may follow :)
What do you think of this ?