If the code is in a library it is not your call to decide what is satisfactory performance and what requirements are. A user of your library could decide that running this code inside JVM (via Jython) is more important than using C extensions for speed.

Real world example, an excellent tool -- Universal Feed Parser http://www.feedparser.org/

  # If a real XML parser is available, feedparser will attempt to use it.  feedparser has
  # been tested with the built-in SAX parser, PyXML, and libxml2.  On platforms where the
  # Python distribution does not come with an XML parser (such as Mac OS X 10.2 and some
  # versions of FreeBSD), feedparser will quietly fall back on regex-based parsing.
  try:
      import xml.sax
      xml.sax.make_parser(PREFERRED_XML_PARSERS) # test for valid parsers
      from xml.sax.saxutils import escape as _xmlescape
      _XML_AVAILABLE = 1
  except:
      _XML_AVAILABLE = 0
      def _xmlescape(data,entities={}):
          data = data.replace('&', '&')
          data = data.replace('>', '>')
          data = data.replace('<', '&lt;')
          for char, entity in entities:
              data = data.replace(char, entity)
          return data