As far as I understand, @gplopes uses Mamp with the default Apache server? And you, @Theo, only experience this issue on your Raspberry, but not on your VPS, both running Nginx, right? Have you tested running your Raspberry with Apache yet?
When exactly are the images generated, before you actually reload the page or really only on the second load?
What about memory settings? Cache settings?
Somehow it does indeed seem as if generating the images was too slow for some reason. But I can hardly imagine this happening with a single file only in a Mamp environment with standard Mamp settings, unless the images are really huge.
However, the whole purpose of generating thumbs asynchronously in Kirby 3 is to circumvent those memory issues that can happen when generating thumbs for many images or generating many image sizes.