Mention that seek and tell over a TextIOWrapper can be very slow.
diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index a97fae0..26ddd8f 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -814,6 +814,8 @@
binary I/O over the same storage, because it implies conversions from
unicode to binary data using a character codec. This can become noticeable
if you handle huge amounts of text data (for example very large log files).
+Also, :meth:`TextIOWrapper.tell` and :meth:`TextIOWrapper.seek` are both
+quite slow due to the reconstruction algorithm used.
:class:`StringIO`, however, is a native in-memory unicode container and will
exhibit similar speed to :class:`BytesIO`.