Instagram Downgrades Video Quality for Less Viewed Clips
Briefly

"In general, we want to show the highest quality video we can when someone is watching a Story or Reel [...] But if something isn't watched for a long time, because the vast majority of views are in the beginning [after initial posting], we will move to a lower quality video, and then if it's watched again a lot, then we'll re-render the higher quality video."
Read at Social Media Today
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