No, the CPU doesn't explicitly throttle back in any case, AFAICT. (Well, except that any DSP tasks force it to stay out of 400 MHz, because the clock multiplier magic means that the DSP actually runs slower at 330 MHz CPU. Think of it as hauling a trailer, and disabling overdrive. Or better yet, don't.)

Lock screen and keys does just that, and kills the backlight (for me, with timeouts equal; see note above).
Softpoweroff, as far as I can tell, does the same, but also (depending on settings) goes offline, preventing any battery drain from wireless or bluetooth traffic. (But perhaps increasing battery drain, if you were listening to music over BT, as it then switches the speakers amplifier on instead...)

But, whenever the CPU has little to do, it shifts down (since OS2008), and of course stops the clock entirely when it has nothing to do (as always).