It has never taken any longer than 1.5 days for dry CYA to dissolve in my pool and test to the expected result. I use a product by Seabrite (I think that's the name); it's flakes rather than granules, maybe that makes a difference. The most I've ever added were 2 pounds directly into the skimmer with the pump running, of course. I can't remember now whether I had to adjust the pH after the addition but if I did, it's not a big deal.

If the Leslie's product doesn't have any other "magic" stuff in it, like copper, for instance, I see no reason not to use it other than cost. It's likely pricey goods.