Lack of fusion (LoF) defect remains a critical challenge in overlap laser beam welding of austenitic stainless steels, particularly in high-speed welding applications such as fuel cell production. This defect is governed by keyhole-driven melt transfer across the inter-sheet gap, yet the influence of welding speed on this process remains poorly understood. In this study, the influence of welding speed on keyhole behaviour and melt pool dynamics during laser overlap welding with an inter-sheet gap was examined using high-speed synchrotron X-ray imaging. Experiments were conducted on AISI 304 sheets at welding speeds of 4, 8, and 12 m/min. Imaging at 100 kHz enabled direct visualisation of keyhole fluctuations and melt transport, allowing quantitative evaluation of melt bridge formation, lifetime, and reformation behaviour across the gap. The results reveal that welding speed governs melt bridge dynamics through regime-dependent keyhole and melt pool dynamics, leading to nonmonotonic variation in LoF severity. The highest gap bridging capability was observed at intermediate welding speed, where melt bridge formation was more frequent and bridge growth was enhanced. In contrast, the lower speed produced short-lived bridges and long reformation intervals, while the higher speed promoted frequent but discontinuous bridge formation, both contributing to increased lack of fusion.