The downloader is based on a peer-to-peer transfer service. If it was just released this morning, you're waiting for yourself and others to trickle down with more of the data. The patch is also large (over 1gb) so it will take a while, espec on a slow connection.
Make sure you go to View (top menu), and go to the options, and check "don't throttle in the background". If the box is unchecked, then the download will slow if you're doing something else and will have to reconnect to more peers when focus is back on the downloader.
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