If the UFH has half the output of radiators, then it makes sense that it would take twice as long to warm up.
It's really far more complicated than you suggest, and my quick calculations only scratched the surface.
The 'heating up time' is only a small part of the story - the fact that the floor surface eventually comes to a constant temperature of 25° (my assumption) in no way necessarily means that the room will be adequately heated.
When, after the 'heating up', thermal equilibrium is eventually achieved, the equilibrium temperature of (the air in) the room will depend upon the rate at which heat is being put into the room and the rate at which heat is being lost from the room (through walls, doors, windows ceilings etc.) -
regardless of how that heat is supplied.
Hence, if a radiator had to have a certain heat output (rate of heat transfer) in order to heat the room adequately in the 'equilibrium state', then UFH which supplied only half of rate of heat transfer would not heat the room adequately, even when in that equilibrium state. That's simple physics.
Where the argument falls down is to say that leaving the heating on all day is somehow going to be hugely costly. Afterall, it's on all day, but at half the power input. Therefore the only additional cost is the losses during the part of the day when the heating would have been off, but is on.
As above, if, in the equilibrium ('heated up') state, the UFH cannot provide heat at the rate that a radiator would have to in order to heat the room adequately, then the room would not be heated adequately by UFH, even in that equilibrium state. The fact that UFH takes a long time to get to that equilibrium state merely adds insult to that injury.
The Laws of Physics being what they are, and assuming 'losses' are much the same in all cases, maintaining a certain room temp for 24/7 will require the same amount of energy (hence cost) over any period of time, regardless of how that heat is supplied.
But of course during that time when the rads are given up heat and gone cold, the higher thermal mass of the concrete floor (with insulation below it...) is still giving up heat and upon your return hasn't cooled down anything like as much as the house WOULD had done if the rads were in service.
Yes, if, after a period during which the UFH has been off, the surface temp of the floor remains a bit above 'ambient' temp by virtue of stored heat released by the concrete, that would to some extent speed up the 'heating up' process, but that would not have any effect on the adequacy/inadequate of the UFH to heat the room. I haven't a clue as to how much heat the concrete might store (less the inevitable 'losses'), but I suspect this would not be a major consideration.
Kind Regards, John