Context

The ocamlopt -O3 flag is currently used only for binaries, but if we care about performances, it should be applied everywhere.

One cannot set env only in the toplevel dune file because dune-project files prevent inheritance.

Setting env in the toplevel dune-workspace file doesn't have that limitation.

Manually testing the MR

Document the interface of any function added or modified (see the coding guidelines)
Document any change to the user interface, including configuration parameters (see node configuration)
Provide automatic testing (see the testing guide).
For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
Select suitable reviewers using the Reviewers field below.
Select as Assignee the next person who should take action on that MR