From the video I saw, it appeared that both ends of the span collapsed at the same time - no slow motion - and immediate.
Must be a different video from the one that I studied -- frame-by-frame -- using diffferential transparency mode to reveal minute changes in position. According to that video, the collapse took a full quarter-minute, with the deck collapse over the south main piers occurring only after the main span was completely in the water and the "splash wave" had subsided.
If you have a URL, please share it with me. I'd love to study the video you saw -- thanks!