The purpose of the wall stat is not to control the temperature of the water inside the rad, it is to control the temperature of the room. The room gets warmer by the hot rad giving out its heat into the room.
If you run the rads at a cooler temperature (which you can do by turning down the BOILER temperature by say 10 degrees) it WILL take longer for the rooms to warm up.
Given your comment above, that would indeed mean turning the boiler setting down from 3 to 2, say.