If you use the rsync:// protocol scheme (i.e. when you connect to a rsyncd daemon) then no encryption will be used (although password authentication is done using a MD4-based challenge-response system and is probably still reasonably secure).
If you use the hostname:/some/path scheme then rsync transparently calls SSH, which encrypts everything, and uses SSH’s native authentication mechanisms. As far as I can tell, some OpenSSH versions supported an option Ciphers null in the configuration file, but this has been removed in later versions.
Generally you shouldn’t worry about encryption overhead, unless you are working in a 1 Gbit network or you have old computers.