I think it probably did dry too quick, but it also looks to me, as if the mix was too strong, (too much cement, not enough sand). See if you can remove it, (chip it off), clean/dampen the area, and coat it with a 4 to 1 mix, sand and cement render,, neatly finished off with a sponge,,, it will certainly look a lot better than it does now.
Roughcaster.