Image optimisation
Sending an image no bigger than it will be shown, in a format that compresses well, with its size stated in the page. Images are often the largest thing a page downloads, so this can be the cheapest big win going.
An agent asked to add a picture adds the file you gave it: a 4000-pixel photo, in a box 400 wide. Nothing complains, and on a fast connection nobody notices. So tell it the display size and the format as part of the job, because neither shows up in a test.
Say it to your agent
“hero.png is 4000 pixels wide and shown at 800. Generate 800 and 1600 wide versions in AVIF and WebP, serve them with srcset and sizes in a picture element with a JPEG fallback, and set width and height. Show me the bytes before and after.”