@walter-born … R click on the video event and from the window that opens, select Insert/Remove envelope. From the sub-window that opens, check Velocity. A green line then appears on the event.

At the far L edge of the event, there is node (keyframe) which defaults at 100% speed, but R click the node and a whole lot of speed options appear in a window, including Set To which allows you to code you own speed. Double-L clicking at any spot on the green velocity allows to set a new speed from that point. If you want to gradually reduce speed of an event from 100% to say 50%, then add a new node at the point where you want the slow-down to start, and then another node at the point where you want the 50% speed achieved, and then R click that second node to select/code the desired speed. Voila!