... in CPython, but is it guaranteed to be faster in e.g. PyPy? We should be very careful about making any performance promises/points unless we know it is fairly universal that the design will explicitly make something faster or slower relative to another operation.

To me this PEP is entirely a question of whether the operators will increase developer productivity and not some way to do dict merging faster, and so performance questions should stay out of it unless it's somehow slower than dict.update().