diff --git a/src/ChangeLog b/src/ChangeLog
index e537848ea41..b3d2f64bcc7 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,9 @@
+2011-07-25  Eli Zaretskii  <eliz@gnu.org>
+
+	* xdisp.c (compute_display_string_pos): Fix logic of caching
+	previous display string position.  Initialize cached_prev_pos to
+	-1.  Fixes slow-down at the beginning of a buffer.
+
 2011-07-24  Eli Zaretskii  <eliz@gnu.org>
 
 	* xfaces.c (check_lface_attrs) [HAVE_WINDOW_SYSTEM]: Allow `nil'
diff --git a/src/xdisp.c b/src/xdisp.c
index 8b164008c3f..7493fbff008 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -3137,7 +3137,7 @@ next_overlay_change (EMACS_INT pos)
 /* Record one cached display string position found recently by
    compute_display_string_pos.  */
 static EMACS_INT cached_disp_pos;
-static EMACS_INT cached_prev_pos;
+static EMACS_INT cached_prev_pos = -1;
 static struct buffer *cached_disp_buffer;
 static int cached_disp_modiff;
 static int cached_disp_overlay_modiff;
@@ -3184,18 +3184,22 @@ compute_display_string_pos (struct text_pos *position,
 	  && BUF_MODIFF (b) == cached_disp_modiff
 	  && BUF_OVERLAY_MODIFF (b) == cached_disp_overlay_modiff)
 	{
-	  if (cached_prev_pos
+	  if (cached_prev_pos >= 0
 	      && cached_prev_pos < charpos && charpos <= cached_disp_pos)
 	    return cached_disp_pos;
 	  /* Handle overstepping either end of the known interval.  */
 	  if (charpos > cached_disp_pos)
 	    cached_prev_pos = cached_disp_pos;
 	  else	/* charpos <= cached_prev_pos */
-	    cached_prev_pos = max (charpos - 1, BEGV);
+	    cached_prev_pos = max (charpos - 1, 0);
 	}
 
       /* Record new values in the cache.  */
-      cached_disp_buffer = b;
+      if (b != cached_disp_buffer)
+	{
+	  cached_disp_buffer = b;
+	  cached_prev_pos = max (charpos - 1, 0);
+	}
       cached_disp_modiff = BUF_MODIFF (b);
       cached_disp_overlay_modiff = BUF_OVERLAY_MODIFF (b);
     }