Okay, I see what you mean concerning performance.
Anyway, I would really like my featured images to be enlargened beyond the margin to the original size of the image. It is true that the featured image is right now 739 x 554px – but the original image I have uploaded is 1138px. If I in the post specifics specify: Align Featured Image – Single Page: ‘Align Wide’ and Page Featured Image Size – Single Page: ‘Full’ – it seems to be rendered even wider (1305px). How should I specify to keep the width 1138px?