Lazy loading means not requesting an ad until the reader scrolls near it. It is one of the few changes that improves the reader's experience, the sponsor's results and your costs at the same time — while making one of your headline numbers look worse.

What improves

Viewability rises sharply. If the ad is only requested as it approaches the viewport, nearly every impression served is one that got seen. Viewability commonly moves from around 50% to 80%+.

The page gets faster. Fewer requests compete with your initial render. Below-the-fold ads stop delaying the content people came for.

Costs fall. If your ad server bills per request, you stop paying for ads nobody saw.

What gets worse

Your impression count drops, often by 20–40%, because you stop counting the ads that were never seen.

Those impressions were always worthless. But if you sell on a CPM basis, you were being paid for them.

The decision

If you sell flat-rate monthly sponsorships, lazy load everything below the fold. There is no downside. Same fee, better viewability, faster site, happier sponsor.

If you sell on CPM, think about it. You are choosing between a larger number of worse impressions and a smaller number of better ones. The honest position is the second, and it is a good argument for moving to flat rate.

Never lazy load the top slot

The unit above the fold is visible immediately. Delaying it means a visible gap during load and a slower LCP. Load it eagerly, always.

A good default: eager for anything in the first screen, lazy for everything else.

Getting the threshold right

The trigger distance matters. Too tight and the reader sees an empty box while the ad loads. Too loose and you are back to requesting ads nobody reaches.

Around 200–400px before the slot enters the viewport works well. On a fast connection that is enough time for an image to arrive before it is scrolled to.

Reserve the space regardless

Lazy loading and layout shift are separate problems. Even a lazily loaded slot must have its dimensions reserved in CSS, or the ad arriving will shove content down at the exact moment someone is reading it.

Telling your sponsor

Worth a sentence, because their impression count will be lower than they might expect from your traffic:

Ads load as readers scroll to them, so the impressions I report are ones that were actually on screen. It means the count is lower than raw pageviews and the viewability is much higher.

Most sponsors prefer that once it is explained. The ones who do not are buying impressions rather than results, and they were going to be difficult anyway.