Photo book editor for an online print shop

Your online print shop sells photo books. Customers upload 50 to 300 photos, 3 to 25 MB each, and arrange them on pages in a browser editor. The editor must feel quick on an ordinary laptop. People often work on a book over several evenings.

A book has 20 to 80 square pages, 30 cm a side. The printer needs one PDF for each book, rendered at 300 dpi (dots per inch). A photo with too few pixels for its printed size comes out blurry, and customers ask for refunds. About 2,000 books are ordered a week, and 10 times as many are started.

What does the editor load, what do you save as the customer works, and how is the print file made?