Using the extension has obvious plusses. If you have to keep within a data limit, anything you can do to compress pages is to be welcomed. Compressing pages can also have the happy side effect of speeding up load times. Sounds great, yeah? What's not to like about optimized pages? Look to the extension's page in the Chrome Web Store and the description says:
When this extension is enabled, Chrome will use Google servers to compress pages that you visit before downloading them. SSL and incognito pages will not be included.Very much the same information is provided on the relevant Google support page:
When you're using Data Saver, Google's servers reduce the amount of data that gets downloaded when you visit a webpage. How much data gets saved depends on the type of content on the page. Secure pages (pages that start with https) and incognito pages load without any data savings.
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