Index: Lib/_pyio.py
===================================================================
--- Lib/_pyio.py	(révision 88332)
+++ Lib/_pyio.py	(copie de travail)
@@ -1488,6 +1488,7 @@
         self._decoded_chars_used = 0  # offset into _decoded_chars for read()
         self._snapshot = None  # info for reconstructing decoder state
         self._seekable = self._telling = self.buffer.seekable()
+        self._b2cratio = 0.0
 
         if self._seekable and self.writable():
             position = self.buffer.tell()
@@ -1655,7 +1656,12 @@
         # Read a chunk, decode it, and put the result in self._decoded_chars.
         input_chunk = self.buffer.read1(self._CHUNK_SIZE)
         eof = not input_chunk
-        self._set_decoded_chars(self._decoder.decode(input_chunk, eof))
+        decoded_chars = self._decoder.decode(input_chunk, eof)
+        self._set_decoded_chars(decoded_chars)
+        if decoded_chars:
+            self._b2cratio = len(input_chunk) / len(self._decoded_chars)
+        else:
+            self._b2cratio = 0.0
 
         if self._telling:
             # At the snapshot point, len(dec_buffer) bytes before the read,
@@ -1709,20 +1715,51 @@
         # forward until it gives us enough decoded characters.
         saved_state = decoder.getstate()
         try:
+            # Fast search for an acceptable start point, close to our
+            # current pos
+            skip_bytes = int(self._b2cratio * chars_to_skip)
+            skip_back = 1
+            assert skip_bytes <= len(next_input)
+            while skip_bytes > 0:
+                decoder.setstate((b'', dec_flags))
+                # Decode up to temptative start point
+                n = len(decoder.decode(next_input[:skip_bytes]))
+                if n <= chars_to_skip:
+                    b, d = decoder.getstate()
+                    if not b:
+                        # Before pos and no bytes buffered in decoder => OK
+                        dec_flags = d
+                        chars_to_skip -= n
+                        break
+                    # Skip back by buffered amount and reset heuristic
+                    skip_bytes -= len(b)
+                    skip_back = 1
+                else:
+                    # We're too far, skip back a bit
+                    skip_bytes -= skip_back
+                    skip_back = skip_back * 2
+            else:
+                skip_bytes = 0
+                decoder.setstate((b'', dec_flags))
+
             # Note our initial start point.
-            decoder.setstate((b'', dec_flags))
-            start_pos = position
-            start_flags, bytes_fed, chars_decoded = dec_flags, 0, 0
-            need_eof = 0
+            start_pos = position + skip_bytes
+            start_flags = dec_flags
+            if chars_to_skip == 0:
+                # We haven't moved from the start point.
+                return self._pack_cookie(start_pos, start_flags)
 
             # Feed the decoder one byte at a time.  As we go, note the
             # nearest "safe start point" before the current location
             # (a point where the decoder has nothing buffered, so seek()
             # can safely start from there and advance to this location).
-            next_byte = bytearray(1)
-            for next_byte[0] in next_input:
+            bytes_fed = 0
+            need_eof = 0
+            # Chars decoded since `start_pos`
+            chars_decoded = 0
+            for i in range(skip_bytes, len(next_input)):
                 bytes_fed += 1
-                chars_decoded += len(decoder.decode(next_byte))
+                chars_decoded += len(decoder.decode(next_input[i:i+1]))
                 dec_buffer, dec_flags = decoder.getstate()
                 if not dec_buffer and chars_decoded <= chars_to_skip:
                     # Decoder buffer is empty, so this is a safe start point.