We use dispersion engineering to control the signal propagation speed in the feed lines of superconducting single-photon detectors. Using this technique, we demonstrate time-division-multiplexing of two-pixel detectors connected with a slow-RF transmission line, all realized using planar geometry requiring a single lithographic step. Through studying the arrival time of detection events in each pixel vs the fabricated slow-RF coplanar waveguide length, we extract a delay of 1.7 ps per 1 μm of propagation, corresponding to detection signal speeds of ∼0.0019c. Our results open an important avenue to explore the rich ideas of dispersion engineering and metamaterials for superconducting detector applications.