Responsive vs fixed-size units

Responsive vs fixed-size units
Responsive ad code is the default and it's right 90% of the time — let it fill the slot.
BUT fixed beats it in two spots: above the fold where you don't want layout shift killing your CLS score, and in a tight sidebar where responsive collapses to a sad 300x250 anyway.
Lock those two to fixed sizes. Leave everything in-content responsive so it grabs the fat 336x280 / leaderboard fills.
Mixing them per-slot beats picking one religion.
Test it on one page first.