Markup_Tree is already scalable;

--- a/src/Tools/jEdit/README	Fri Sep 10 15:05:30 2010 +0200
+++ b/src/Tools/jEdit/README	Fri Sep 10 15:16:51 2010 +0200
@@ -26,9 +26,6 @@
     previous commands (proof end on proof head), or markup produced by
     loading external files.
 
-  * Some performance bottlenecks for massive amount of markup,
-    e.g. when processing large ML sections.
-
   * General lack of various conveniences known from Proof General.
 
 Despite these shortcomings, Isabelle/jEdit already demonstrates that