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I just ran the telco benchmark ...
 http://www.bytereef.org/mpdecimal/quickstart.html#telco-benchmark
... on _decimal to see how the PEP-393 changes affect the module.
The benchmark reads numbers from a binary file, does some calculations
and prints the result strings to a file.
Average results (10 iterations each):
Python 2.7: 5.87s
Revision 1726fa560112: 6.07s
Revision 7ffe3d304487: 6.56s
The bottleneck in telco.py is the line that writes a Decimal to the
output file:
 outfil.write("%s\n" % t)
The bottleneck in _decimal is (res is ascii):
 PyUnicode_FromString(res);
PyUnicode_DecodeASCII(res) has the same performance.
With this function ...
 static PyObject*
unicode_fromascii(const char* s, Py_ssize_t size)
{
 PyObject *res;
 res = PyUnicode_New(size, 127);
 if (!res)
 return NULL;
 memcpy(PyUnicode_1BYTE_DATA(res), s, size);
 return res;
}
... I get the same performance as with Python 2.7 (5.85s)!
I think it would be really beneficial for C-API users to have
more ascii low level functions that don't do error checking and
are simply as fast as possible.