I had similar experiences. It seems whatever compression they use is fast but not as good as TinyPNG or pinga.
This is a workaround, not a fix, but it works:
- Upload your image and let Publii resize it.
- Find your resized files (e.g. `C:\Users\whatever\Documents\Publii\sites\whatever\input\media\posts\1\responsive` )
- Optimize those resized files instead of the original.
As long as you don’t upload a new image, Publii won’t regenerate the responsive ones.