> It’s not 100 % clean either, but much better. Granted, this is WebP re-encoding of an already lossy compressed JPEG, so we stack 2 steps of destructive compression. But this is what Google Page Speed insights encourage you to do and what a shitload of plugins enable you to do, while pretending it’s completely safe. It’s not.
He's re-encoding the JPEG compressed images. That is a huge mistake.