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?
Python is very slow at handling large amounts of non-string data. For some operations, you may see that it is 1000 times slower than C/C++, so yes, you should investigate into this and do necessary benchmarks before you make time-critical algorithms in Python.
However, you can extend python with modules in C/C++ code, so that time-critical things are fast, while still being able to use python for the main code.
Performance of Python worth the cost?