The UFH I fitted was fitted in the main to dry the floor after a shower so wheel chair did not slide on the tiles. It cooled off within seconds of the shower being switched on, and took 20 minutes to re-heat and mother was not going to wait 20 minutes after her shower so it was a complete failure. We also made a mistake the fall was too great so chair rocked, and the sculptured tiles instead of increasing grip, reduced the grip because they retained the water.
As time went by the thermostat was replaced about 4 times, and in the end the sensor got stuck in the pocket so only the room temperature of the thermostat worked. It was not worth removing the tiles to fit a new pocket and although it could in theroy still be used, it is now never switched on.
Also it was found because of the limit in tile temperature the UFH on its own could not heat the room with the required extractor fan running. Yet the towel rail was good enough without the UFH being switched on.
OK it was in a wet room, but it was a complete failure. It did have loads of insulation fitted below the UFH and as a result the tiles are not really that cold even without the UFH being used. On tests it took about 20 minutes to feel the heat touching the floor with ones hand, it would take about an hour for the floor to reach full working temperature. Since unlike rest of house the floor in wet room was well insulated so never gets as cold as quarry tiles used in other parts of the house it is not really possible to say without use of shower how long it takes to cool down. But within 20 minutes the tiles are only warm not hot.
The whole idea with the exception of storage heaters is for a heating system to warm up and cool down quickly. A radiant heater does not need to warm the air so must be the most economic form of heating, however the problem is control. With other forms we measure how economic by used the heat up and cool down times. So a fan heater may heat the room to required temperature is 10 minutes, if the room is used for 1 hour 40 minutes then that is 90% efficient, use an oil filled radiator in same room then 20 minutes to warm up so 80% efficient. But used for a longer time then efficiency goes up. For UFH to heat the room looking at hours not minutes but still with 24 hour heating it is still efficient it is only with short term heating when it fails.
Room size and air changes clearly also matter. The major problem is UFH has a limit to how warm it can make the floor so one can still stand on the floor so the differential between heat of floor and heat of air is rather small so air changes can cool the room down quicker than the floor can heat it up. So step one with bitter experience has to be to install heat recovery units so air changes do not cool the room too much. Had I fitted heat recovery units to the wet room it may have worked better.