Average Compression 18.56%, photos still too large

Hello,
So we downloaded ShortPixel Image Optimizer because our speed score on Google is very slow, and it said that images were one of the major problems. We subscribed in order to optimize all of our photos.

However, despite having run the optimize and getting the message that optimization was successful, images are still one of the major things that drag down our site. ShortPixel says that average compression is 18.56%. Any ideas why images are still such a problem? Is it the backups perhaps?

https://www.ads-software.com/plugins/shortpixel-image-optimiser/