Kiln of the first flame. might not be as fast but has each knight drops a different chunk and it's a 100% drop rate. One of the farther back ones, 4th or 5th always drops it.
It's a lot like the question with the DWs and the regular Chunks/Slabs, do you want your guaranteed Chunks (if so go to the Kiln) or do you want to possibly come up empty completely on some runs but have at least the chance at a red/regular Slab (if so, then Izalith/New Londo is where you wanna go).
It takes way too long to farm the guys in Izalith, and the chances of a slab together with the benefits of one make it not worth going for in my opinion. The second Black knight in the Kiln always drops one and if you have Homeward it goes by a lot faster.