The concept of a creature with a "metabolic rate" rate too slow to detect seems like nonsense to me

My point was not that the processes might be too slow to detect, but that it could be too slow for us to correctly identify them for what they were. The problem is not in the physics or chemistry, but in us. After all, its not likely to be a particular well funded area of research is it - "stare at this rocklike thing for five years and let me know if any happens, will you?" *grin*