Add docs explaining that Read::bytes is slow.

Author: nnethercote

library/std/src/io/mod.rs

@@ -969,7 +969,10 @@ pub trait Read {
     ///
     /// The default implementation calls `read` for each byte,
     /// which can be very inefficient for data that's not in memory,
-    /// such as [`File`]. Consider using a [`BufReader`] in such cases.
+    /// such as [`File`]. Consider using a [`BufReader`] in such cases. Even
+    /// then, going through the [`BufReader`] for every byte will still be
+    /// relatively slow. If speed is important, consider instead reading the
+    /// input in larger chunks.
     ///
     /// # Examples
     ///