I had all of the images on my site optimized perfect according to Google PageSpeed Insights for the last year+. Google PageSpeed Insights can be found here: https://developers.google.com/speed/pagespeed/insights/
However, I just ran it for my site and noticed practically all the PNG, JPG, JPEG images are now showing up as needing to be Optimized under the "Should Fix" section. As in "Compressing could save x KB (x%)".
As usual they still allow you to download the images as the compressed versions that they suggest to use, but some don't appear to be losslessly compressed and are showing quality loss.
Did Google come up with a new image compression algorithm to use for its PageSpeed Insights? If so, then optipng and jpegoptim in Linux are severely outdated to whatever compression algorithm that Google is now using in PageSpeed Insights. So, can't even do a mass compression on an images folder in Linux.
It looks like both big (top 500) sites and small sites were all hit by this and its sending many sites into the red on PageSpeed Insights due to "Optimize Images" suggestion.
Not sure if it's a Google bug, or what is going on, looking to see others thoughts on this as I'm sure other Webmasters will notice soon.
Same for me, I looked since ages to find someone raising the issue, thanks to you, I feel less alone.
Seems now images are compared to with-loss compression, as when you download the "optimized" version of the images from pagespeed insight and compare with original image, you can see some loss.
It's not very fair from them but maybe they compare with a new algorithm that find the loss nearly invisible. Difficult for us to do the same.
I use initially jpg with 90% quality to create the image, maybe they want less.