In many cases, forum posts contain images inserted via bbcode. With pages showing 10 or most posts per page, it doesn't make much sense to load all these images at once, but rather only when scrolling down.
This could be especially beneficial for initial page load.
Therefore, the img bbcode should generate an img tag with attribute loading="lazy".
Browsers are increasingly supporting this: https://caniuse.com/#feat=loading-lazy-attr