You can treat a thermistor as a thermal mass coupled to your system
through a heat connection of limited capacity. If the pins connect to
another system or board at a different temperature then it also has
heat losses and functions as a heatflow meter of sorts. If there is self
heating then some heat is generated in the thermistor and adds to the heat
flow from other sources.

The response speed depends on the specific heat coefficient of the
thermistor and the inflow/outflow capacities of the thermal coupling and
of the losses.

In practice one measures the time required for the whole assembly
including the heatsing to heat or cool as the thermistor's transient
response is very fast compared to it.

Peter

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