--bwlimit=KBPS
This option allows you to specify a maximum transfer rate in
kilobytes per second. This option is most effective when using
rsync with large files (several megabytes and up). Due to the
nature of rsync transfers, blocks of data are sent, then if
rsync determines the transfer was too fast, it will wait before
sending the next data block. The result is an average transfer
rate equaling the specified limit. A value of zero specifies no
limit.