Patch 3.3 is upon us and Blizzard prepares to enable the Blizzard downloader to spread files in the background. Make sure to enable background downloading using the Launcher.

Blizzard Quote:
The next major content patch is coming and in order to smooth the way, we will shortly enable background downloading of most of the data for this patch. Players who use the Launcher and have background downloading enabled will automatically start receiving the data. The download speed is overall faster than before, so you may want to adjust your background download settings. You can adjust these settings in the Launcher, by visiting the options page. If you want the fastest possible download, check the “Don’t throttle background download” option. However, note that this choosing that option can affect the performance of other programs running at the same time.