Facebook Developers explain how they include ~200 byte preview JPEG images in user profile JSON payload to speed up load times
Facebook profiles can be slow to download and display. This is especially true on low-connectivity or mobile networks, which often leave you staring at an empty gray box as you wait for images to download. This is a problem in developing markets such as India, where many people new to Facebook are primarily using 2G networks. Our engineering team took this on as a challenge: What could we design and build that would leave a much better first impression?
How a change in preview photos helped speed up profile and page loads by 30 percent.
Read more from the source: Facebook Code