It's a legacy deck so it's definitely not slow, it can kill on turn 2. It's main issue is that it kills through 1 combo or not and all you have to protect your combo are 4 Cabal Theory which although incredibly powerful aren't that much protection on their own, especially with how fragile this deck is vs graveyard removal. A single
Surgical Extraction is often enough to make it almost impossible to win for the deck and a single
Deathrite Shaman can make things very difficult and many decks play the shaman because it's so insanely good.
Anyway, what you want it to do is discard and get things in the graveyard, somewhat clumsy to start off since you always want to start second. With some luck you get a Phantasmagorian to discard so you can activate it's ability, react to it by activating it's ability again allowing you to discard 6 cards. You then most likely have a dredge creature in your graveyard so next turn you can start flipping your own library into your graveyard and you can get creatures into play through Narcomoeba, Nether Shadow and Ichorid which in turn you sacrifice to flashback Dread Return to cast Balustrade Spy which in the absence of lands turns your entire deck over into your graveyard.
With your entire deck in your graveyard and a mix of the aforementioned 3 creatures and the tokens they generate through Bridge From Below you can cast more Dread Returns for Flayer Of The Hatebound and then Golgari Grave-Troll, the troll deals damage equal to it's power to an opponent and since it's probably 35/35 or stronger you oneshot your opponent.