I assume that Gatsby generates the different widths/formats at build time. How well does that scale? Like if you have a thousand images, what's the build time like?

We have a job framework that can distribute image processing jobs on supported platforms* as well as automatically caching subsequent jobs. So it scales easily and as most builds don't have that many image changes (if any), the impact on warm builds is negligible.

* like Gatsby Cloud https://www.gatsbyjs.com/products/cloud/