lazy loading: progressive vs on-demand

progressive: Loads the visible image as soon as the page is displayed and the other ones after everything else is loaded in the background ("loads the visible slides on init, and then progressively loads the rest of the slides on window.load()."). Should be used if the other images will be used most (or all) of the times the page is displayed.

on-demand: Loads the visible image as soon as the page is displayed and the other ones only when they're displayed. ("[...] loads slides on demand. When a slide becomes visible (or on the before slide callback) the load is fired.") Should be used if the other images of the carousel are displayed very rarely.

Source: https://github.com/kenwheeler/slick/issues/35, especially jasonday's comment from 5. Apr

It's worth mentioning that there is another value for the lazyLoad option in the source code, not documented though: 'anticipated'. It was introduced in release 1.7.1

lazyLoad accepts 'ondemand', 'progressive', or 'anticipated' for lazy load technique.

'ondemand' will load the image as soon as you slide to it.

'progressive' loads one image after the other when the page loads.

'anticipated' pre-loads the 1 next and 1 previous image.