pylon:
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.