I think the 24 grit would be ideal to lap other coarse stones because below about 300 grit you really start to cause major damage to diamond plates. I have not yet owned the 24 but I had the 60 grit and used it long enough to know how it works.
The 60 grit came with a smooth lapped top that would hardly cut into steel, after a lapping with other coarse stones the surface became coarser but the binder was so tough it did not allow the abrasive to release and replenish itself. You also had to about stand on the blade for any abrasive release to happen.
The only things it ever really worked on was a old hatchet and a large Japanese hatchet, seemed as if you needed thick heavy steel or the stone was about useless. On these large blades the stone released abrasive quickly but produced some nasty deep scratches that about ruined the shinogi line of the japanese hatchet and with the rapid dishing made an overly round bevel on the American style hatchet. I spent almost an hour on the japanese hatchet with a 120 Bamboo stone (crazy fast stone, wears crazy fast too) hardly getting all the scratches out and the other hatchet I took to the belt sander because the 60 did more damage than good.
So yes, the 60 grit was slower than my 150, 180, 220 and several other coarse stones for two reasons. First, if it is any faster with a given blade all that saved time will be lost in removing the scratches it produces. Second, it's binder was too hard, more like the type of binder used with lapping stones, this in itself will make a stone slower.