Thank you Will!
As you suspect, a large collection of images can be an issue. Unfortunately, the cache described in this article won't work out of the box in production, because the images are not committed to Git (the tutorial explicitly gitignore them). So the images will be generated every time... impossible with 500+ images.
Workaround 1: Simply commit your images. In other words, un-gitignore their directory. It somehow breaks the build-at-deploy spirit, yet this is a completely legit approach.
Workaround 2: Much harder, because it's documented for Netlify and not Vercel... The idea is to store the image data and build time (eg. the title, the author name, etc.) and let a function generate the images on demand. It's all here: blog.philippebernard.dev/automated-social-images-…