I was having awful download speeds while downloading game using BlizzardDownloader, like less than 30kbps, even when i had 10MB connection.
So i discovered that BD have an issue when you have installed any virtual ethernet, more specific, VMWare Virtual Network Adapters. For your connection download at decent speed, you MUST turn off/disable all these adapters.
After i did and let only the real one, my download speed went full speed to heavens.
Hope that help someone that had same problem but nothing on blizzard faq could help.