The thermos is a double-walled vessel with the air evacuated from the space between the walls.

The thermos is a double-walled vessel with the air evacuated from the space between the walls. Why does the temperature of the liquid poured into the thermos change very slowly?

Because the air is pumped out almost completely (some of it still remains), therefore, the thermal conductivity of a highly discharged gas decreases sharply, which means that thermal energy is transmitted through it extremely slowly. Therefore, hot or cold liquid poured into a thermos changes its temperature very slowly, from 8 to 24 hours.