Image smacking is an internal Pressidium service that optimises your media in a lossless way. It is enabled by default and automatically performs a number of optimisations on each individual image.
You can find each media's achieved optimisation rate by visiting the Media Library in your WordPress administration menu, and selecting the List icon on the left. You can also manually smack an individual image.
We strongly suggest you keep the image smacking enabled. If you want to disable it for some reason, or do bulk smacking, please open a support request so we can investigate your case.