The image that was four megabytes because nobody looked

A hero image uploaded at 6000 pixels wide, served at 1200, on every page load, for eleven months.

$ identify hero.jpg
hero.jpg JPEG 6000x4000 4.1MB

$ wp media regenerate --only-missing

# and the thing that should have caught it:
add_filter( 'big_image_size_threshold', fn () => 2560 );
# which is core behaviour since 5.3 and had been
# disabled by a plugin.

WordPress has scaled large uploads down since 5.3 and a plugin had turned it off for a reason nobody could reconstruct. The general lesson is that the largest asset on a page is worth checking after any plugin change, because a filter somewhere can silently undo a platform default — and four megabytes on a mobile connection is the whole page budget.