Last week, I wondered whether Google’s new data compression feature for Chrome for iOS was partially motivated by the inability to use the Nitro engine to speed up page load times. Today, I have stumbled upon the technical document that details how the data-saving process actually works – in short, it uses Google’s proxy to optimize web traffic sent by Chrome.

The proxy server receives the request initiated on the mobile device, initiates a request for the required resource on your behalf, and then optimizes each asset before delivering it back to the client. The content optimization is performed by our open-source PageSpeed libraries, which are specifically tuned for the Chrome Mobile browser. The rendering of the page, and all JavaScript execution, is performed by the client’s browser.

Of note, the transcoding of images from JPEG and PNG to WebP:

Over 60% of the transferred bytes, for an average page, are images. Hence, the proxy takes great care to optimize and transcode all images to the WebP format, which requires fewer bytes than other popular formats, such as JPEG and PNG. The proxy supports the new WebP lossless format for certain images, and also optimizes the perceptual quality of each image based on device screen resolution and pixel density of your device.

I’ve never been a fan of speed optimization through proxies personally, but I’m curious to try out Google’s implementation. The feature is still rolling out for Chrome users on iOS.