The longitudinal spin Seebeck effect describes the transfer of a spin current from a magnetic insulator driven by a temperature gradient. Here we present our recent studies on the longitudinal spin Seebeck effect on the picosecond timescale in both ferromagnets and antiferromagnets using THz emission spectroscopy. As a function of temperature, we observed a different temperature dependence compared to DC electrical studies carried out in the same temperature range. By comparing different antiferromagnets belonging to the same family of fluoride perovskites we are able to correlate the spin transfer efficiency with the bands of the magnon spectrum.