I have the same problem. Only ~300K/s download speed.
In parallel i need to download an update for another game. 1.2GB was downloaded within less than 10 minutes. The download speed
of ESO did not goes down while doing this.
So it is not MY internet connection or my PC or the weather or whatever you want to tell us.
Moreover, here in Europe you will easily find other forums with threads complaining about this issue, too.
So i would assume that Lycrates is right^^
If we will see these download "down-under-speed" when updating the game later that this will be not funny.
And i'm going to became afraid about latency IN the game :/
/regards
D.