Vertical comic chapters for a reading app

Your app publishes vertical comics, read by scrolling down on a phone. An artist delivers each chapter as one long PNG image, 800 pixels wide and up to 60,000 pixels tall, about 40 MB. The app has 1.5 million readers. A new chapter of a popular series is read by 200,000 people in its first hour.

New chapters go live at a set time, often 10:00 on a Saturday. Many readers use phones with 3 GB of memory on slow mobile data. A reader should see the first panels within 2 seconds of opening a chapter. Scrolling at reading speed should never show a blank gap.

What files do you make from an artist's chapter, and how does the app load them as the reader scrolls?