Possible values for effectiveType are 'slow-2g', '2g', '3g', or '4g'. On slow connections this capability allows you to improve how quickly pages load by serving lower-quality versions of resources.
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Adaptive Serving using JavaScript and the Network Information API
Possible values for effectiveType are 'slow-2g', '2g', '3g', or '4g'. On slow connections this capability allows you to improve how quickly pages load by serving lower-quality versions of resources.
Full article