Why does it take so long to compute the RPSD. It is taking 10 times as
long as the PSD analysis itself.
Theory Manual Equation 19.141 is used in computing RPSD. The number of calculations involved is approximately proportional to modes**3 depending on the resolution set with STORE,PSD. For a large number of modes the number of calculations becomes very large. My suggestion is to reduce the resolution on the STORE,PSD command to STORE,PSD,3 or so. Then once the RPSD is computed integrate it; take the square root and compare it to the 1 sigma value in POST1 to ensure the accuracy of the RPSD curve. While speed is a major limitation of RPSD , it does allow the user flexibility in computing the RPSD of any dof, stress or force quantity after the solution is already done. In some software the user has to specify the desired result prior to solution. 

