You'll know if Chrome is optimizing a page because an icon will appear in front of the URL, and you'll have the option to load the original version if you can stand to. Google says it will use the feature sparingly -- only when loading the full page would be "painful to users." In the company's eyes, that means if the network is 2G or slow-2G, or if Chrome estimates the page will take more than five seconds to load any content given network conditions and device capabilities. In the US, where we generally have higher network speeds, this might not happen too often, but it could have a big impact in areas where LTE is still being built out.
In a blog post, Google explains that when it optimizes HTTPS pages, only the URLs will be shared with the company, keeping cookies, logins and other data private. To be honest, it's a little surprising the Lite browsing mode wasn't already available for HTTPS pages, which now make up so much of the web. And if you're browsing on a PC, Chrome Lite is still limited to HTTP pages. It's also important to note that if you're using Incognito mode, Data Saver won't work at all, and it's not currently available on iPhones or iPads. There's certainly some room for improvement, but if you're an Android user, you might have a reason to turn on Data Saver in Chrome settings.