If this is real (I am a Millennial but I don't actively watch/use Tiktok to have seen it), I think that it might be a learned habit from using tape and old slow digital cams. Especially tape since it does need to get moving. Though old digital cameras also have a much more noticeable delay than basically any modern smartphone. Go watch old internet videos and you will also hear the camera person say "go/okay, go" like miming how shows and movies are clapped. Could it be trimmed? For sure.