Just a still image could be a meteor or space junk reentering. Would have to see a video to determine the speed. A plane would probably appear to be going slower than a meteor or space junk.
If you go to Flightaware you can see the plane accelerated from 598 to 613 MPH in last few minutes. Its last reported speed is the fasted recorded of the entire flight. I thought that was strange when I first saw this.
Looks like it was on fire and still moving horizontally.