A lot of people, including us, have been getting really slow download speeds on the OS X El Capitan, or macOS Sierra, download. Chances are, the servers nearest to you are being seriously hammered, hence the slow download.
An easy fix to this is to change your DNS to either use OpenDNS or a free DNS from the land down under (Australia)
Go to System Preferences > Network > Advanced > DNS remove any existing DNS entries and add either of the following:
Pause your download, give it a minute and then restart it.
Bandwidth consumption jumped to almost completely consume the connection we had immediately.