Obviously in this case it is your internet connection, but for the lot of people who are not familiar with battle.net app and have connections of like 25Mbps or higher, if you have this kind of an issue, you need to check the settings, and set the download rates to 0. This will clear up any issues with the download being capped out at a much lower rate than what your internet is capable of. Changing it to 0 makes it unlimited, meaning it will go as fast as your connection will let it.
