Given the same number of pipeline stages and the same manufacturing node (say, 65 nm) and the same voltage, simple devices should run faster than more complicated ones. Also, merging multiple pipeline stages into one should not slow down by a factor grater than the number of stages.
Now take a five-year-old CPU, running 14 pipeline stages at 2.8 GHz. Suppose one merges the stages; that would slow down to below 200 MHz. Now increase voltage and reduce number of bits per word; that would actually speed things up.
That's why I don't understand why many currently manufactured microcontrollers, such as AVL, run at abysmal speed (such as 20 MHz at 5 V), even though far more complicated CPUs manufactured years ago were capable of running 150x faster, or 10x faster if you roll all pipeline stages into one, at 1.2 V-ish. According to the most coarse back-of-the-envelope calculations, microcontrollers—even if manufactured using borderline obsolete technology—should run at least 10x faster at one quarter of the voltage they are supplied with.
Thus the question: What are the reasons for slow microcontroller clock rates?