Loose compression for all images directly from Drupal I need to do image loose-compressing usually done with GIMP/TinyPNG/GPI, but just inside Drupal, but also For all images, in a single process or several sporadic processes (say, for each content type).

This could save much time instead of download, compress, reupload, etc.

Is there a way to loose-compress all images in the application so that webpages containing them load faster?

If you don't know such a tool but does know a way to show all nodes that has images, please comment naming such a tool, so it could at least help in the alternative of downloading and manually loosely-compressing each image before re uploading it.
You're looking for this but in D8 - https://www.drupal.org/project/imageapi_optimize. You can still use the tools to do it manually. 

PNG - Use https://pngquant.org/ first with these settings --speed=1 --quality=90-99 --force --ext .png and then https://github.com/subzey/zopfli-png
JPG - https://github.com/mozilla/mozjpeg and then https://stackoverflow.com/questions/5579183/jpegtran-optimize-without-changing-filename

EDIT: This new D8 project just appeared https://www.drupal.org/project/file_processor