The X-ray Integral Field Unit (X-IFU) is an instrument of European Space Agency’s future NewAthena space observatory, with the goal to provide high-energy resolution (<4 eV at X-ray energies up to 7 keV) and high-spatial resolution (9 in.) spectroscopic imaging over the X-ray energy range from 200 eV to 12 keV, by means of an array of ∼1500 transition-edge sensors (TESs) read out via superconducting quantum interference device time-division multiplexing (TDM). A TDM-based laboratory test bed has been assembled at Netherlands Institute for Space Research, hosting an array of 75×75 μm2 TESs that are read out via 2-column × 32-row TDM. A system component that is critical to high-performance operation is the wiring harness that connects the room-temperature electronics to the cryogenic readout componentry. We report here on our characterization of such a test bed, whose harness has a length close to what was envisioned for X-IFU, which allowed us to achieve a co-added energy resolution at a level of 2.7-eV full width half maximum at 6 keV via 32-row readout. In addition, we provide an outlook on the integration of TDM readout into the X-IFU focal plane assembly development model.