I think awhile back one of the experts here posted something on using a slurry stone to create a slurry and I believe the consensus was don't do it. Even though the slurry stone might be softer than the hone what might be happening is pieces of the slurry stone might be breaking off but the actual minerals in the stone could be harder than your hone and could be messing up your grit schedule.
I've been told nogura stones are like chalk but you really don't know. Afterall steel is really not that hard on the hardness scale in comparison to most minerals so a coticule with garnets which has a hardness of about 6 or so on the moh scale compares to steel with a hardness between 3-4 maybe. The Norton is faster because its harder, its ceramic. So its not just the grit size but the material thats why a diamond stone cuts so fast.