http://lists.gnu.org/archive/html/bug-bash/2012-11/msg00034.html

From 530d4988afd68ea9d2cf1b0267d4dc821d0d204f Mon Sep 17 00:00:00 2001
From: Mike Frysinger <vapier@gentoo.org>
Date: Mon, 19 Nov 2012 17:58:51 -0500
Subject: [PATCH] bash: speed up `read -N`

Rather than using 1 byte reads, use the existing cache read logic.
This could be sped up more, but this change is not as invasive and
should (hopefully) be fairly safe.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
---
 builtins/read.def | 21 ++++++++++++++++-----
 externs.h         |  1 +
 lib/sh/zread.c    | 15 +++++++++++++--
 3 files changed, 30 insertions(+), 7 deletions(-)

diff --git a/builtins/read.def b/builtins/read.def
index e32dec7..81a1b3f 100644
--- a/builtins/read.def
+++ b/builtins/read.def
@@ -457,7 +457,10 @@ read_builtin (list)
   interrupt_immediately++;
   terminate_immediately++;
 
-  unbuffered_read = (nchars > 0) || (delim != '\n') || input_is_pipe;
+  if ((nchars > 0) && !input_is_tty && ignore_delim)
+    unbuffered_read = 2;
+  else if ((nchars > 0) || (delim != '\n') || input_is_pipe)
+    unbuffered_read = 1;
 
   if (prompt && edit == 0)
     {
@@ -505,10 +508,18 @@ read_builtin (list)
 	  print_ps2 = 0;
 	}
 
-      if (unbuffered_read)
-	retval = zread (fd, &c, 1);
-      else
-	retval = zreadc (fd, &c);
+      switch (unbuffered_read)
+	{
+	case 2:
+	  retval = zreadcn (fd, &c, nchars - nr);
+	  break;
+	case 1:
+	  retval = zread (fd, &c, 1);
+	  break;
+	default:
+	  retval = zreadc (fd, &c);
+	  break;
+	}
 
       if (retval <= 0)
 	{
diff --git a/externs.h b/externs.h
index 09244fa..a5ad645 100644
--- a/externs.h
+++ b/externs.h
@@ -479,6 +479,7 @@ extern ssize_t zread __P((int, char *, size_t));
 extern ssize_t zreadretry __P((int, char *, size_t));
 extern ssize_t zreadintr __P((int, char *, size_t));
 extern ssize_t zreadc __P((int, char *));
+extern ssize_t zreadcn __P((int, char *, int));
 extern ssize_t zreadcintr __P((int, char *));
 extern void zreset __P((void));
 extern void zsyncfd __P((int));
diff --git a/lib/sh/zread.c b/lib/sh/zread.c
index 5db21a9..af7d02b 100644
--- a/lib/sh/zread.c
+++ b/lib/sh/zread.c
@@ -101,15 +101,18 @@ static char lbuf[128];
 static size_t lind, lused;
 
 ssize_t
-zreadc (fd, cp)
+zreadcn (fd, cp, len)
      int fd;
      char *cp;
+     int len;
 {
   ssize_t nr;
 
   if (lind == lused || lused == 0)
     {
-      nr = zread (fd, lbuf, sizeof (lbuf));
+      if (len > sizeof (lbuf))
+	len = sizeof (lbuf);
+      nr = zread (fd, lbuf, len);
       lind = 0;
       if (nr <= 0)
 	{
@@ -123,6 +126,14 @@ zreadc (fd, cp)
   return 1;
 }
 
+ssize_t
+zreadc (fd, cp)
+     int fd;
+     char *cp;
+{
+  return zreadcn (fd, cp, sizeof (lbuf));
+}
+
 /* Don't mix calls to zreadc and zreadcintr in the same function, since they
    use the same local buffer. */
 ssize_t
-- 
1.7.12.4