I got feedback that isolating ads from your page through <iframe> is desirable, but that the impact on browser load UI, the load event, and "fast back", are very much not desirable.
It seems there should be some way to indicate that the <iframe> should function, but is not a critical part of the page. Perhaps this could be tied to sandboxing in some way. This is somewhat similar to https://w3c.github.io/resource-hints/ but that seems rather indirect and does not include UI or "fast back" considerations. -- https://annevankesteren.nl/