We report a series of in-situ monitoring experiments on the high-speed laser directed energy deposition (HSL-DED) with a discernible distance between the identified laser spot and melt pool boundary, termed melt pool lag, during the high-speed laser-substrate interaction. The experiments investigated the melt pool lag feature on both mild steel and 300M high-strength steel substrates at processing speeds from 5 to 20 m/min with and without powder delivery. The analysis of results indicates that the correlation between melt pool lag measurements and several deposit characteristics could shorten the parametric investigation cycle in HSL-DED. The melt pool lag measurement is positively proportional to the processing speed at a given incident laser power level. Further examinations also found that substrate material properties significantly impact melt pool lag for a given set of process conditions. The melt pool lag measurements obtained from the mild steel substrate was significantly higher than that on the 300M high-strength steel substrate. An analytical model on melt pool formation has been developed with temperature-dependent thermophysical data of substrate materials to estimate the melt pool lag in HSL-DED. The observation of melt pool lag feature and the proposed analytical model enhance the understanding of the melt pool dynamics in HSL-DED. They also provide insights to improve powder utilisation during HSL-DED deposition.