I mean saving after rendering. There is no possibility to save the image in the webp. And conversion after all the work, after post-processing, is again an additional waste of time, which I don’t have much. Anyway thanks for the advice!
You are right, conversion to webp is an additional job after all the post-processing, but it can be done automatically as a batch job.
You just have to convert images to webp and if you wish, videos to webm, but there is no need to modify any script files if you keep original file extensions. Ren'Py does not care if the file extension does not match the file type. You can convert compression to webp, but keep png extensions and the game will still work.
You can use xnconvert for the batch conversion, for example. You just have to keep file extensions.