Radioshack sold a voice recognition chip in the 80s[1] that was a simple 8-bit microcontroller. If you are willing to slip on the accuracy and false positives you can do recognition with very little computation.
the downside was power usage. Motorola made one that was power efficient, used by nokia in the 90s and its pretty much the same chip in google's phone line today (just even more power efficient).
[1] http://21stdigitalhome.blogspot.ca/2013/06/vcp200-voice-reco...