Lossless optimization is a one-shot deal generally. With JPGs you will see no improvement on repeated attempts. With PNGs, you may see a couple bytes, but not much more than that, and after a couple passes, you’re unlikely to see anything new.
One thing that is very important if you want the best lossless rsults is to make sure you tell EWWW to remove metadata. Your images may not have any, but it is an oft overlooked savings opportunity.
With lossy compression, you may see additional savings on every pass, but you will lose quality every single time there is savings achieved. The savings can vary wildly from image to image, and I never recommend more than 1 pass with lossy optimization.
All that said, if you want to experiment with lossy optimization and multiple passes, just make sure you have a good backup before you get too far.
