"More ad slots = more money"

"More ad slots = more money"

True until it isn't, and the turn is sharper than people admit.

Adding units raises Page RPM right up to the point where each new slot cannibalizes the bid pressure on the others (auction pressure is finite per pageview) and tanks your viewability score — which advertisers price in. Below ~50% viewability on a unit, CPMs drop and that slot starts dragging the average.

🚩 Hidden variable: viewability-weighted demand. Premium buyers cap spend on low-viewability inventory, so unit #11 doesn't just earn little — it lowers what units #1-10 fetch by polluting your site's quality signal.

Not saying density doesn't help — saying past the viewability cliff you're trading next quarter's CPM for this week's impression count.