Your dad is likely right (as usual? :-)). With modern equipment, slow but steady wins the race.
I assume we're talking about a modern high efficiency condensing gas boiler. If that's not the case and you still have an old-fashioned conventional unit, please do us all a favour and have that replaced first thing in the morning. (If you happen to be in Europe, that replacement will likely pay for itself before this winter is over.)
As the name suggests, "condensing" boilers achieve their super high efficiency by cooling the combustion products down below the dew point, so that the vapour produced by combustion condenses to liquid water, releasing a lot of heat in the process. The exhaust from such a boiler then has roughly the temperature of human breath. In order to do this, the boiler has to work with return water temperatures no higher than about 50 degrees Celsius.
Thing is, radiator systems were commonly sized to old non-condensing systems which work with much hotter water. Because the power output of a radiator scales with water temperature, running the radiators colder means you have to keep them running for much longer to deliver enough heat to the room.
At the same time, modern boilers use burners capable of throttling all the way down to perhaps 20% of their maximum power. At install time, the boiler is programmed with a curve telling it exactly how much heating power is needed to compensate the losses of your house at given outdoor temperature, and the boiler uses it to produce exactly enough heat to keep interior temperature stable.
This is useful because the boiler is typically the most efficient in this low-power, deeply throttled regime (where it produces the least amount of combustion gases, so they stay longer in contact with the heat exchanger and have enough time to condense very thoroughly).
In contrast to this, old boilers used to know only two states: "off" and "full blast". Because "full blast" had to be powerful enough to keep you from freezing when the temperature goes below -20 °C once in a half century, the only option to stay comfortable at other times was to use a thermostat to switch the boiler on and off.
Unfortunately, bang-bang control mixes poorly with the modern kind of boilers with modulated power. Remember that the boiler is calibrated to deliver just enough heat to make up for losses? Guess what happens if the termostat is configured for a big nighttime set-back, so the heating cuts out in the evening. Your house gets cold overnight and when the termostat turns heat on in the morning, the temperature stays uncomfortably cold until noon or later as there's very little excess power to raise it. Depending on how smart your boiler is, it might realize that this is going on and turn into a high-power mode (or you will get unhappy with how cold your house is and switch it by hand to a more aggressive curve). Either way, with the boiler asked to deliver lots of heat in a short time, water temperatures will go way up, above the condensation point, significantly compromising efficiency. (This effect is even more prononunced with heat pumps: Those excel at the low-temperature slow-n-steady mode, but if they realize they can't keep up with the demand, they'll go into full panic mode and turn on fallback resistive heating elements, giving you 1) a nice warm and cozy home, and 2) a heart-stopping power bill).
By the way, there's nothing wrong with thermodynamics: Domestic heating has much more to do with human perception than thermodynamics. With radiators staying only mildly warm to touch all day, you will likely feel more comfortable even at a lower room temperature than with bang-bang control. That's because when the heating cuts out completely, you lose two things: 1) radiant heat transfer from the radiator, and 2) air circulation. With radiators off, air in the room will stratify (warm air will end up under the ceiling while the floor will be much colder, especially around windows). Humans feel much colder if their feet are cold, even if the average temperature in the room is still okay.