Selecting AVR microcontroller with or without PicoPower?
PicoPower AVRs are not necessarily slower. If you want them to run on low voltage, only then you have to run them slower. 

Older "V" devices tend to be inferior in sense of speed, newer PicoPower as far as my experience goes (I'm building my first low power design now) can run as fast as "normal" devices when powered from 5V.