Of course it depends on what you are doing, but 10x is a pretty good case. I recently re-wrote a C++ tool in python and even though all the data parsing and computing was done by python libraries that wrap high performance C libraries, the program was still 6 or 7 times slower than C++. Had I written the python version in pure python (no numpy, no third party C libraries) it would no doubt have been 1000x slower.