Hi,
Thanks for reaching here.
There is a basic difference between both method of lazy loading,
Progressive: These methods are load images at the beginning of the page in the background like loads the visible slides on init ( Start before displaying ), and then progressively loads the rest of the slides on window.load().
Ondemand: These methods are load images at the real-time of slides transition and real-time clicks as loads slide on demand. When a slide becomes visible (or on the before slide callback) the load is fired.
Please let me know if you have any other queries.
Thanks