Oversized images are the most common reason a site feels slow, and the easiest thing to fix.
A 4000 pixel photograph shown in a 600 pixel column is downloaded in full and then thrown away by the browser. The visitor waits for every one of those pixels and sees none of them, which is pure cost.
Twice the displayed width is the sensible target. That covers high density screens, where two device pixels are used for each layout pixel, and going beyond it buys nothing anyone can see.
WebP typically saves another quarter to a third over JPG at the same quality, and is supported by every current browser. For a site with many images that adds up quickly.
Around 80 per cent suits photographs. Flat graphics and screenshots often need more, or a different format.
WebP for smaller files and broad modern support. JPG if you need to support very old software.
No, which also means your unpublished assets stay unpublished.