Chemical reactions slow down when the temperature of the reactants is lowered. Developing and etching are both chemical processes.
If the board was exposed properly and all the unwanted etch resist was removed during developing, then the problem was obviously in the etch process.
I have found that heating the etchant is pretty much required for the board to etch in a timely manner. I preheat my FeCl3 etchant (in it's storage container) in a water bath to about 130F and the etch goes rather quickly.
Actual etch time depends on several factors: temperature of the etch bath, size of board and amount of copper to be removed, etchant saturation (age of the etchant), etc.