C++ Advantage of Proxy Classes

Scott Meyers says in "Effective Modern C++" that the expression

Matrix sum = m1 + m2 + m3 + m4

(where all objects have type 'Matrix') "can be computed much more efficiently if operator+ for Matrix objects returns a proxy for the result instead of the result itself. That is, operator+ for two Matrix objects would return an object of a proxy class such as Sum instead of a Matrix object.".

Now, I understand that proxy classes emulate behaviour of some other classes and do implicit converisons, but how and why is proxy approach in this case more efficient?

Best regards