qbopt(P5) will be small if |P5(jw)| is small around the frequency of the modulus of the right-half plane pole.
qFor poles with the same modulus, bopt(P5) will be smaller for a plant with relatively larger real part poles than for a plant with relatively larger imaginary part poles(i.e., a pair of real right-half plane poles has a much worse effect on the performance than a pair of almost pure imaginary axis right-half plane poles of the same modulus).