Loose compression for all images directly from Drupal
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