I'm looking at implementing a fuzzy logic controller based on either PyFuzzy (Python) or FFLL (C++) libraries.
I'd prefer to work with python but am unsure if the performance will be acceptable in the embedded environment it will work in (either ARM or embedded x86 proc both ~64Mbs of RAM). 
The main concern is that response times are as fast as possible (an update rate of 5hz+ would be ideal >2Hz is required).  The system would be reading from multiple (probably 5) sensors from an RS232 port and provide 2/3 outputs based on the results of the fuzzy evaluation.
Should I be concerned that Python will be too slow for this task?
From your description, speed should not be much of a concern (and you can use C, cython, whatever you want to make it faster), but memory would be. For environments with 64 Mb max (where the  OS and all should fit as well, right ?), I think there is a good chance that python may not be the right tool for target deployment.
If you have non trivial logic to handle, I would still prototype in python, though.
Performance of Python worth the cost?