Update: here is a document explaining
why HTTPS is faster than HTTP.
It boils down to a Google-developed additional session layer (SPDY) from which only HTTPS profits. Basically it is not HTTPS vs. HTTP but multiplexed sessions over a single TCP connection versus unmultiplexed sessions. It would be possible to do HTTP over SPDY too (it is just not done). HTTP2 is basically SPDY standardised and further developed.
In principle HTTP is slightly faster than HTTPS: there are caching facilities so that not every retransmission has to be originated by the client. HTTPS lacks that because relaying stations cannot read what they transmit.
How SPDY speeds up things is especially via the session multiplexing. This gains lots of time because of the delayed TCP-ack, which takes 500ms.
bakunin