Dependence of the mean interpolation time spent per frequency per direction at spatial coordinate point on the total number of frequency points in one PRD line profile. Times are given for the forward (▸) and the backward (◂) transforms for three different algorithms: general (red), precomputed (blue), and precomputed equidistant (green) interpolation. The measurements were done for the Mg ii h&k lines in the 1D Bifrost model atmosphere.