I'm not a radiant heat pro, but in my system, which is under wood flooring, not in a slab, we have to run it at about 140 degrees to provide adequate heat. Concrete is much more thermally massive, and therefore shouldn't need as high of a temperature, I would guess in the 120-125 degree range. You should be targeting about 80 degrees surface temperature of the floor, generally to maintain comfortable room temperature. Is your system controlled by surface temperature, or room air temperature? Usually your pumps will have different speed settings as well, which changes the temperature requirements... higher speed requires higher heat to get same gain, but spreads the heat more evenly through the zone if that is an issue, lower speeds pull more heat out of the water per cycle, but you can end up with the beginning of the loop warmer than the end.
You want consistent heat, so your system should not heat up and drive the temperature up quickly, then stay off for a long time. It should cycle on and off relatively frequently to maintain a constant slab temperature. You will probably need to adjust your outgoing water temperature until you reach this state.
Hopefully someone on here with more knowledge of this will chime in with more specifics, my knowledge is quite limited.