I crush them into my mash/sparge water before I start heating, often the night before, and they don't dissolve well for me (water is around 55F usually). I always have a ring of particles around the rim of the vessel.
Crushing with a mortar and pestle or just with a spoon inside of another spoon will get it powdered, but I think it dissolves better in hot water. You can also just get powdered sodium metabisulfite, it's the same thing. (although some might be potassium metabisulfite instead, but it's the anion that you care about anyway, so it makes no difference).
Your idea of dissolving in hot water first might be a good and easy test to do for yourself. I'll throw one in some boiling water tonight to see if it dissolves more quickly.