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iobench benchmarking tool showed that the UTF-8 encoder is slower in Python 3.3 than Python 3.2. The performance depends on the characters of the input string:
 * 8x faster (!) for a string of 50.000 ASCII characters
 * 1.5x slower for a string of 50.000 UCS-1 characters
 * 2.5x slower for a string of 50.000 UCS-2 characters
The bottleneck looks to be the the PyUnicode_READ() macro.
 * Python 3.2: s[i++]
 * Python 3.3: PyUnicode_READ(kind, data, i++)
Because encoding string to UTF-8 is a very common operation, performances do matter. Antoine suggests to have different versions of the function for each Unicode kind (1, 2, 4).