Fix bug #14838 with slow scrolling with certain fonts.

 lisp/simple.el (default-font-height): Don't call font-info if the
 frame's default font didn't change since the frame was created.

lisp/ChangeLog

+2013-07-13  Eli Zaretskii  <eliz@gnu.org>
+
+	* simple.el (default-font-height): Don't call font-info if the
+	frame's default font didn't change since the frame was created.
+	(Bug#14838)
+
 2013-07-13  Leo Liu  <sdl.web@gmail.com>
 
 	* ido.el (ido-read-file-name): Guard against non-symbol value.

lisp/simple.el

@@ -4739,10 +4739,15 @@ lines."
 
 (defun default-font-height ()
   "Return the height in pixels of the current buffer's default face font."
-  (cond
-   ((display-multi-font-p)
-    (aref (font-info (face-font 'default)) 3))
-   (t (frame-char-height))))
+  (let ((default-font (face-font 'default)))
+    (cond
+     ((and (display-multi-font-p)
+	   ;; Avoid calling font-info if the frame's default font was
+	   ;; not changed since the frame was created.  That's because
+	   ;; font-info is expensive for some fonts, see bug #14838.
+	   (not (string= (frame-parameter nil 'font) default-font)))
+      (aref (font-info default-font) 3))
+     (t (frame-char-height)))))
 
 (defun default-line-height ()
   "Return the pixel height of current buffer's default-face text line.