i guess that depends on the math involved.

that is, will it be more power efficient to go at Ghz, but for a shorter time, or at Mhz, but longer time?

in the end, it ends up a question of watts/second, and how many seconds are needed to complete a task.

but i guess thats under a idealized, single task scenario. Ones multitasking and such gets involved, one also have to ponder how long it takes to get in and out of a given task, and if there ever will be a point where the system can sleep as all tasks are waiting for some kind of event.

i guess thats where a dsp and similar comes in, as its basically a kind of specialized multicore (hell, one can even claim that with a GPU). As such, it can perform a encoding or decoding task at much lower watt/second then the cpu.