Why is rsync -avz faster than scp -r?

Rsync will obviously be faster than scp if the target already contains some of the source files, since rsync only copies the differences. But I suspect your question was about doing a straightforward copy to an empty target. You've passed the option to ; this turns on compression. If the network bandwidth is the limiting factor (it often is), compression can improve the transfer speed by a noticeable amount. You can also enable compression with by passing the option. This should about even thin…