Rated frequencies and power requirements
TDP is for designing/choosing the heatsink - it is the maximum power that the CPU can dissipate, the value is usually the same for the whole range of CPUs (so that you can use any CPU with the same cooler).

Now, the power actually dissipated is proportional to the frequency (that's why some CPUs slow down when not under load) and the square of the voltage (that's why when you increase the voltage while overclocking the CPU gets much hotter). So, yes, a slower CPU will use a bit less power.

This is about PC CPUs, but it is probably true for microcontrollers as well.