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`.