This plugin seems to work better than Smush goes. It took one of my sites from a 12 score to a 99!
reSmush.it : the only free Image Optimizer & compress plugin
reSmush.it is the best WordPress image compression plugin. It allows you to automatically optimize images on upload as well as offer a bulk optimization option for older images.
It uses the reSmush API to optimize images and allows you to choose the optimization level for your uploads. The downside is that it doesn’t have different compression levels. It also limits optimization to uploads lower than 5MB in size. You can exclude individual images from compression in case you need to add an image in full size.