Weldability of high-strength steels is strongly influenced by welding thermal cycles, particularly through their effect on heat-affected zone (HAZ) microstructure, hardness, and susceptibility to hydrogen-assisted cracking. In this study, six commercially available high-strength steels (S355MC, S500MC, S700MC, S960MC, S960QL, and S1100MC) were investigated using Gleeble physical simulation to reproduce welding thermal cycles with cooling times t8/5 = 5-20 s. Microstructural characterization, hardness measurements, and instrumented Charpy impact testing were performed and complemented by Tekken weldability tests. The results show that thermomechanically controlled processed (TMCP) steels (S355MC, S500MC, and S700MC) exhibit stable transformation behavior, maintaining consistent hardness and toughness across the investigated cooling range. In contrast, steels with yield strengths of 960 MPa and above exhibit a narrow processing window, where rapid cooling promotes the formation of hard martensitic microstructures, while slower cooling leads to grain coarsening and reduced impact toughness. Instrumented Charpy testing revealed a significant decrease in absorbed energy and crack resistance with increasing cooling time. The findings demonstrate that weldability in ultra-high-strength steels cannot be reliably assessed based on hardness alone. A combined evaluation of cooling time, microstructural evolution, hardness, and fracture behavior is required to support the selection of appropriate preheating and welding conditions.