What is the meaning of a low processing speed index score on the wais-iii?

Depends on how low the score. Processing speed is basically a measure at how quickly and efficiently one can perform cognitive tasks, i.e. recognition stimuli. That does not mean the person with a lower processing speed score is not capable of correctly identifying the proper information, it just means it will take that person a little longer on average to than it would someone with a higher processing speed score.