It's easy to clog the uplink of DSL and cable modem connections when uploading a large amount of data to a remote host. Even though you won't be using your (normally large) downlink capacity during such a transfer, your connection will still seem quite slow if you let rsync go as fast as it can, because outgoing TCP packets such as HTTP requests will have to compete with your transfers for bandwidth on your uplink.
To get around this, you can use --bwlimit to give your uplink a little breathing room. For example, to limit the bandwidth to 10000 kilobytes per second, you might do something like this:
rsync --bwlimit=10000 -a dir host:destination_dir