Increasing the power of modern solid-state lasers allows higher speeds for laser beam welding. In particular, full penetration welding enables the production of butt joints, which are typically used in the production of continuously manufactured tubes for example. However, increasing the welding speed affects the flow conditions of melt and metal vapor. In general, welding speeds above 8 m/min are well known to cause imperfections, especially spatter formation on the sheet top side. In contrast, the melt pool behavior and the mechanisms of spatter formation on the sheet bottom side are largely unknown. Therefore, monitoring the processing by means of high-speed synchrotron X-ray imaging expands the understanding of the mechanisms causing spatter formation on both sheet sides welding the high-alloy steel AISI 304. Image processing was used to measure the keyhole front inclination and the keyhole length at different positions in addition to a visual evaluation of the highly dynamic process behavior. The keyhole length increases with higher welding speeds due to a changed melt flow and more spatters detach from the melt pool swelling. Humping occurs on the sheet top side for the welding speed 20 m/min. In addition, a locally increased keyhole front inclination on the sheet bottom side occurs. The position of spatter formation shifts form the melt pool swelling behind the keyhole opening to the keyhole front.