Thin dielectric layers deposited on the inner surface of hollow cylindrical metallic waveguides for Terahertz (THz) waves reduce transmission losses below 1 dB/m. Impact of the dielectric layer on the waveguide dispersion is experimentally investigated by near-field mapping of guided short THz pulses at the input and the output of the waveguide. We obtain dispersion characteristics for the low-loss waveguide modes, the linearly-polarized HE(11) mode and the TE(01) mode, and compare the experimental results to the metallic waveguide dispersion. The additional dispersion due to the dielectric layer is found to be small for the HE(11) mode and the phase velocity is primarily determined by the waveguide radius.