diff --git a/Lib/io.py b/Lib/io.py
index 0c993b1..f543e20 100644
--- a/Lib/io.py
+++ b/Lib/io.py
@@ -1769,20 +1769,20 @@
     def newlines(self):
         return self._decoder.newlines if self._decoder else None
 
-class StringIO(TextIOWrapper):
-    """An in-memory stream for text. The initial_value argument sets the
-    value of object. The other arguments are like those of TextIOWrapper's
-    constructor.
+class _StringIO(TextIOWrapper):
+    """Text I/O implementation using an in-memory buffer.
+
+    The initial_value argument sets the value of object.  The newline
+    argument is like the one of TextIOWrapper's constructor.
     """
 
     # XXX This is really slow, but fully functional
 
-    def __init__(self, initial_value="", encoding="utf-8",
-                 errors="strict", newline="\n"):
-        super(StringIO, self).__init__(BytesIO(),
-                                       encoding=encoding,
-                                       errors=errors,
-                                       newline=newline)
+    def __init__(self, initial_value="", newline="\n"):
+        super(_StringIO, self).__init__(BytesIO(),
+                                        encoding="utf-8",
+                                        errors="strict",
+                                        newline=newline)
         if initial_value:
             if not isinstance(initial_value, str):
                 initial_value = str(initial_value)
@@ -1792,3 +1792,271 @@
     def getvalue(self):
         self.flush()
         return self.buffer.getvalue().decode(self._encoding, self._errors)
+
+try:
+    import _stringio
+
+    # This subclass is a reimplementation of the TextIOWrapper
+    # interface without any of its text decoding facilities. All the
+    # stored data is manipulated with the efficient
+    # _stringio._StringIO extension type. Also, the newline decoding
+    # mechanism of IncrementalNewlineDecoder is reimplemented here for
+    # efficiency. Doing otherwise, would require us to implement a
+    # fake decoder which would add an additional and unnecessary layer
+    # on top of the _StringIO methods.
+
+    class StringIO(_stringio._StringIO, TextIOBase):
+        """Text I/O implementation using an in-memory buffer.
+
+        The initial_value argument sets the value of object.  The newline
+        argument is like the one of TextIOWrapper's constructor.
+        """
+
+        _CHUNK_SIZE = 4096
+
+        def __init__(self, initial_value="", newline="\n"):
+            if newline not in (None, "", "\n", "\r", "\r\n"):
+                raise ValueError("illegal newline value: %r" % (newline,))
+
+            self._readuniversal = not newline
+            self._readtranslate = newline is None
+            self._readnl = newline
+            self._writetranslate = newline != ""
+            self._writenl = newline or os.linesep
+            self._pending = ""
+            self._seennl = 0
+
+            # Reset the buffer first, in case __init__ is called
+            # multiple times.
+            self.truncate(0)
+            if initial_value is None:
+                initial_value = ""
+            self.write(initial_value)
+            self.seek(0)
+
+        @property
+        def buffer(self):
+            raise UnsupportedOperation("%s.buffer attribute is unsupported" %
+                                       self.__class__.__name__)
+
+        def _decode_newlines(self, input, final=False):
+            # decode input (with the eventual \r from a previous pass)
+            if self._pending:
+                input = self._pending + input
+
+            # retain last \r even when not translating data:
+            # then readline() is sure to get \r\n in one pass
+            if input.endswith("\r") and not final:
+                input = input[:-1]
+                self._pending = "\r"
+            else:
+                self._pending = ""
+
+            # Record which newlines are read
+            crlf = input.count('\r\n')
+            cr = input.count('\r') - crlf
+            lf = input.count('\n') - crlf
+            self._seennl |= (lf and self._LF) | (cr and self._CR) \
+                         | (crlf and self._CRLF)
+
+            if self._readtranslate:
+                if crlf:
+                    output = input.replace("\r\n", "\n")
+                if cr:
+                    output = input.replace("\r", "\n")
+            else:
+                output = input
+
+            return output
+
+        def writable(self):
+            return True
+
+        def readable(self):
+            return True
+
+        def seekable(self):
+            return True
+
+        _read = _stringio._StringIO.read
+        _write = _stringio._StringIO.write
+        _tell = _stringio._StringIO.tell
+        _seek = _stringio._StringIO.seek
+        _truncate = _stringio._StringIO.truncate
+        _getvalue = _stringio._StringIO.getvalue
+
+        def getvalue(self) -> str:
+            """Retrieve the entire contents of the object."""
+            if self.closed:
+                raise ValueError("read on closed file")
+            return self._getvalue()
+
+        def write(self, s: str) -> int:
+            """Write string s to file.
+
+            Returns the number of characters written.
+            """
+            if self.closed:
+                raise ValueError("write to closed file")
+            if not isinstance(s, str):
+                raise TypeError("can't write %s to text stream" %
+                                s.__class__.__name__)
+            length = len(s)
+            if self._writetranslate and self._writenl != "\n":
+                s = s.replace("\n", self._writenl)
+            self._pending = ""
+            self._write(s)
+            return length
+
+        def read(self, n: int = None) -> str:
+            """Read at most n characters, returned as a string.
+
+            If the argument is negative or omitted, read until EOF
+            is reached. Return an empty string at EOF.
+            """
+            if self.closed:
+                raise ValueError("read to closed file")
+            if n is None:
+                n = -1
+            res = self._pending
+            if n < 0:
+                res += self._decode_newlines(self._read(), True)
+                self._pending = ""
+                return res
+            else:
+                res = self._decode_newlines(self._read(n), True)
+                self._pending = res[n:]
+                return res[:n]
+
+        def tell(self) -> int:
+            """Tell the current file position."""
+            if self.closed:
+                raise ValueError("tell from closed file")
+            if self._pending:
+                return self._tell() - len(self._pending)
+            else:
+                return self._tell()
+
+        def seek(self, pos: int = None, whence: int = 0) -> int:
+            """Change stream position.
+
+            Seek to character offset pos relative to position indicated by whence:
+                0  Start of stream (the default).  pos should be >= 0;
+                1  Current position - pos must be 0;
+                2  End of stream - pos must be 0.
+            Returns the new absolute position.
+            """
+            if self.closed:
+                raise ValueError("seek from closed file")
+            self._pending = ""
+            return self._seek(pos, whence)
+
+        def truncate(self, pos: int = None) -> int:
+            """Truncate size to pos.
+
+            The pos argument defaults to the current file position, as
+            returned by tell().  Imply an absolute seek to pos.
+            Returns the new absolute position.
+            """
+            if self.closed:
+                raise ValueError("truncate from closed file")
+            self._pending = ""
+            return self._truncate(pos)
+
+        def readline(self, limit: int = None) -> str:
+            if self.closed:
+                raise ValueError("read from closed file")
+            if limit is None:
+                limit = -1
+            if limit >= 0:
+                # XXX: Hack to support limit argument, for backwards
+                # XXX  compatibility
+                line = self.readline()
+                if len(line) <= limit:
+                    return line
+                line, self._pending = line[:limit], line[limit:] + self._pending
+                return line
+
+            line = self._pending
+            self._pending = ""
+
+            start = 0
+            pos = endpos = None
+            while True:
+                if self._readtranslate:
+                    # Newlines are already translated, only search for \n
+                    pos = line.find('\n', start)
+                    if pos >= 0:
+                        endpos = pos + 1
+                        break
+                    else:
+                        start = len(line)
+
+                elif self._readuniversal:
+                    # Universal newline search. Find any of \r, \r\n, \n
+                    # The decoder ensures that \r\n are not split in two pieces
+
+                    # In C we'd look for these in parallel of course.
+                    nlpos = line.find("\n", start)
+                    crpos = line.find("\r", start)
+                    if crpos == -1:
+                        if nlpos == -1:
+                            # Nothing found
+                            start = len(line)
+                        else:
+                            # Found \n
+                            endpos = nlpos + 1
+                            break
+                    elif nlpos == -1:
+                        # Found lone \r
+                        endpos = crpos + 1
+                        break
+                    elif nlpos < crpos:
+                        # Found \n
+                        endpos = nlpos + 1
+                        break
+                    elif nlpos == crpos + 1:
+                        # Found \r\n
+                        endpos = crpos + 2
+                        break
+                    else:
+                        # Found \r
+                        endpos = crpos + 1
+                        break
+                else:
+                    # non-universal
+                    pos = line.find(self._readnl)
+                    if pos >= 0:
+                        endpos = pos + len(self._readnl)
+                        break
+
+                # No line ending seen yet - get more data
+                more_line = self.read(self._CHUNK_SIZE)
+                if more_line:
+                    line += more_line
+                else:
+                    # end of file
+                    return line
+
+            self._pending = line[endpos:]
+            return line[:endpos]
+
+        _LF = 1
+        _CR = 2
+        _CRLF = 4
+
+        @property
+        def newlines(self):
+            return (None,
+                    "\n",
+                    "\r",
+                    ("\r", "\n"),
+                    "\r\n",
+                    ("\n", "\r\n"),
+                    ("\r", "\r\n"),
+                    ("\r", "\n", "\r\n")
+                   )[self._seennl]
+
+
+except ImportError:
+    StringIO = _StringIO