> RFC6520-- WHY THE FUCK DOES THIS EXIST? Because it's too computationally expensive for clients to re-establish SSL sessions...?! Really? My dual core 2.15ghz smart phone begs to differ.
No. It's not about CPU time, but about the time taken to establish a connection due to the TLS and TCP handshakes. I think it's only a single round trip for the TLS part (someone will surely correct me if not) but that's on top of the TCP 3 way handshake, which all adds up. You can't mitigate network latency with a faster CPU.
These are partly the same reasons for http2 by the way. Re-using a single connection means avoiding the TCP and TLS setup happening more than once.
Finally, keeping a connection open for a long time and re-using it goes some small way to avoid revealing as much metadata to snoopers, as does multiplexing a single TLS connection rather than creating many.