https://code.facebook.com/posts/1365439333482197/how-we-built-facebook-lite-for-every-android-phone-and-network

"To achieve an extremely byte-efficient wire protocol, instead of using HTTPS, Lite uses a custom message protocol over TLS (directly over TCP). One of the biggest pain points in a 2G network is that establishing a connection can be very slow; it can take multiple seconds. As most Lite traffic flows over a single connection to the backend, this pain point is mitigated in comparison."