I disagree with your bug theory given that the whole time it is visible in frame is one second real time. No bug flies that fast. Also considering a bug that would fly that fast wouldn’t be flat on one side, as seen when this turns it changes shape. If it were say a bottle fly, it wouldn’t be that fast, and wouldn’t change in slenderness when it turns since flies are basically rounded cylinders. Also take into account when it begins to veer offscreen, it accelerates even faster. Flies fly at a consistent speed unless slowing to land.