Optimising images: the quickest win
On most sites images make up the bulk of a page's weight. So that is where the biggest speed gain sits.
The right size
A photo 4000 pixels wide displayed in an 800-pixel block makes the visitor download five times too much data. Resize before uploading.
The right format
Modern formats such as WebP give the same quality at considerably less weight. Most browsers have supported them for years.
Compression
Between 70 and 85 per cent quality the difference is invisible to the eye and the file is far lighter. Above that you pay weight for nothing.
Lazy loading
Images below the fold only load when the visitor reaches them. On a long page that makes an enormous difference.
What not to do
Do not lazy-load the main image at the top of the page. That is exactly the one the visitor sees first, and delaying it makes your page feel slower.
