JPEG Quality configuration
I tested it on a local server. Now I can say that:
1/ Image is processed at upload ONLY if it is larger than maximum authorized width/height (if it fits in this max size, it is not processed).
In this process, the JPEG quality ratio set up in configuration (admin/config/media/image-toolkit) will be applied.
This first step generates the "drupal original image" with will be used to generate image styles.
2/ In this new process of generating the image styles, the JPEG quality ratio will be applied AGAIN when resizing the thumbnails.

This means that it seems better to keep the JPEG quality ration roughly around 60-80% to have lights pages, but in order to avoid it to be applied twice on the images styles thumbnails displayed in the pages, upload "original" images that are 100% quality JPEG and fit to the max allowed size (so that are not processed at upload).