How do I make this zombie mill deck faster?
You are correct in your assessment that this deck is too slow. You should select one direction and go with it - either Zombie or Mill - rather than trying to do both. I recommend removing the Mill strategy, as it is likely to slow for the format, given the current metagame.

I suggest you look at sites such as mtgtop8.com for reference when building decks. There you will see decks with good winning records played by the top players, and be able to use those as a basis for your deck.  For example, this deck finished in the top 32 at the recent Pro Tour in Hawaii.

As for specific feedback on your deck, Standard is a fairly creature heavy, aggressive format. You are playing expensive removal (Sever the Bloodline) in favor of cheap (Tragic Slip, Go for the Throat (play more of those!)). Black Sun's Zenith is probably bad for you, as many of your creatures are low toughness.

Examine the mana curve of your deck. You can do this pretty easily by using any of the deck building sites (like TappedOut), or by arranging your cards by casting cost. In this deck, you have very few cards that you can cast for one or two mana, and as a result you'll lag behind those decks which can play something on turns one and two.

Hope this helps!