Does the DS18B20 itself have the same temperature as the boiler's sensor? That means, have you ensured that there's a good heat transfer from the copper pipes to the DS18B20?
The copper pipe is wrapped in foam insulation and the DS is tucked under that insulation. It is either touching the pipe, or is within 1mm of it (if I move the insulation to see, then the sensor moves around). So the reaction time may not be fast, but since copper is a much worse insulator than foam, then the temperature under the insulation should be very close to the temp of the water after a few minutes.
I checked around with the IR meter and when the boiler claims its supply is 55°C, then the pipes (many types, sizes, etc.) in the basement were all above 51°C, while the ds18b20 was still registering 42-44°C.
Since this seems to be only my problem I'll try to reinstall the sensors, maybe I'll tape them to the pipes to ensure contact, and add insulation around it.
You don't have good thermal contact between the DS18B20 and the pipe.
Firstly you allow one side of the DS18B20 to _radiate_ heat into the surroundings - the foam is probably highly transparent at
the long IR wavelengths that carry radiated heat.
Secondly you haven't bonded the sensor to the pipe with a thermal compound - so its not conducting heat very well from the pipe.
Thirdly the sensor will be conducting heat away down the wires to the surroundings.
You need to put the sensor and the first few inches of wiring inside a shiny aluminium tube, and bond that to the pipe
thermally, then wrap in foam. The metal tube is shiny - much less radiation loss. The tube conducts heat well to the sensor's
entire surface and wiring.
[edit: its a pity the device doesn't come in a metal can with a lug to screw it down! ]