I performed an experiment recently where I tested the specific heat capacity of different metals in water. Once the metals were removed from a beaker and placed into calorimeter with water, thermal equilibrium occurred very quickly. Why is it that thermal equilibrium is achieved so quick when a calorimeter is used?
Its because you were testing metals. They have high thermal conductivity and hence heat transfers through them quickly to the water. Try the experiment with non-metals, e.g., plastics. You'll find it takes longer to reach thermal equilibrium.
Hope this helps