Render functions, to my knowledge, can’t return a promise.

(And you don’t throw a promise, you return and reject it)

The issue at hand is fully synchronous render functions that, e.g., go through a giant list of objects and call a bunch of other render functions on each one synchronously.

This change allows that one function to finish, and interaction to continue before all the children render.

The new suspense API works by throwing Promises when the data isn't ready yet.

I’m not sure what you mean by “throwing promises”...

You mean returning unresolved promises? Or returning rejected promises? Or just throwing an error?

Literally, _throwing_ a Promise.

React will catch promises thrown during rendering, and treat that as a request to "suspend" the component and come back to it later once the promise has resolved.

No. I mean that we yield between calling different components. We can’t pause a giant blocking call in a render but we can amortise “death by a thousand cuts” across many components.