We use this plugin a lot.

We use WP Smush a lot at Simply Compelling, including on lots of client sites. It works best to have condensed the image down to the size you need beforehand and uses a low memory file, then the condensing from WP Smush helps you save another 10%. I think the file needs to be under 8 MB to get smushed. If you are concerned about page speed, you really want it to at least be under 2 MB. Obviously, to optimize for speed, using fewer images and elements is better. You can further boost speed by using a good host, CDN, and cache.