If you want to speed it up (and you're not worried about format), run dark rituals.  They're the sole reason black used to be the fastest color.  Also, I love tragic slip.  It's usually not too hard to make something die with a black deck, and then tragic slip can take out anything.

If you find that liches sit in your hand while you wait for an artifact, you could toss in a darksteel ingot in place of a swamp. 

Just some ideas - I'm not the most experienced player in the world.

I'd say no more swamps than 22, but maybe try some Leaden Myrs. Get Obliterator out 3rd turn instead of 4th. 

I would also put the removal in before the discard. I actually go 4 Despise, 4 Surgical Extraction, and 4 Doom Blade. Then the Duress and Throat are in the sideboard. Check out my deck titled 'No Creatures For You.' It's pretty similar and you might get some ideas from it.

Also, if you want to test your decks, try out the program Cockatrice. I use it now and it's fantastic. You play against other people online.

Some suggestions:

1) a few more swamps (casting Sheoldred/Lashwrithe feeds off swamps)

2) cut some discard for spot removal (go for the throat/doom blade)

3) 4th obliterator? Lashwrithes best friend =)

Other than that, looks pretty powerful. Tell me how it works