Issue 163201 revealed that MenuBar can be blocked for a long time inside waitFinished() method. See full log at
http://www.netbeans.org/nonav/issues/showattachment.cgi/80568/dump-stack.txt
and seek for this part:

"AWT-EventQueue-1" prio=10 tid=0x080cd800 nid=0x4be7 in Object.wait() [0xb366c000..0xb366e0c0]
   java.lang.Thread.State: TIMED_WAITING (on object monitor)
	at java.lang.Object.wait(Native Method)
	- waiting on <0x6e28c748> (a org.openide.util.RequestProcessor$Task)
	at org.openide.util.Task.waitFinished(Task.java:158)
	- locked <0x6e28c748> (a org.openide.util.RequestProcessor$Task)
	at org.openide.util.RequestProcessor$Task.waitFinished(RequestProcessor.java:799)
	at org.openide.util.Task.waitFinished(Task.java:192)
	at org.openide.loaders.FolderInstance.waitFinished(FolderInstance.java:339)
	at org.openide.awt.MenuBar$LazyMenu$MenuFolder.waitFinishedSuper(MenuBar.java:623)
	at org.openide.awt.MenuBar$LazyMenu.doInitialize(MenuBar.java:581)
	at org.openide.awt.MenuBar$LazyMenu.stateChanged(MenuBar.java:555)
	at javax.swing.DefaultButtonModel.fireStateChanged(DefaultButtonModel.java:333)
	at javax.swing.DefaultButtonModel.setMnemonic(DefaultButtonModel.java:274)
	at javax.swing.AbstractButton.setMnemonic(AbstractButton.java:1548)
	at org.openide.awt.Mnemonics.setMnemonic(Mnemonics.java:279)
	at org.openide.awt.Mnemonics.setLocalizedText2(Mnemonics.java:100)
	at org.openide.awt.Mnemonics.setLocalizedText(Mnemonics.java:137)
	at org.openide.awt.MenuBar$LazyMenu.updateProps(MenuBar.java:512)
	at org.openide.awt.MenuBar$LazyMenu.run(MenuBar.java:524)

This is unfortunate, as it is not only useless, but it provides bad visual experience to those who want to see 
progress on screen to be updated.