Major HTTP/2 (RFC 7540) improvements (in comparison to HTTP/1.1):
multiplexing of requests over a single TCP connection, thus
saving costly TCP handshakes (= time and traffic),
saving costly TLS handshakes (= time and traffic);
more efficient message processing through binary message framing,
compression of HTTP headers.
In CalDAV/CardDAV context, it means that for synchronization of one collection, only one TCP/TLS handshake is required and HTTP headers are transferred compressed. (Body compression using gzip is also supported.)
Many servers are supporting HTTP/2, for instance nginx 1.9.5+ and Apache 2.4.12+.
Our Web sites, including this forum, support HTTP/2, too.