Bug 19275 editor: context menu slow on large files
F2 it's slow, but only the first time after a change. then it's fast afain you make another change - it's slow again
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/compare/JavaHistoryAction.java
ISelection getSelection() {
		return fSelection;
	}
		
	protected IFile getFile(Object input) {
		
		ICompilationUnit cu= null;
		if (input instanceof ICompilationUnit)
			cu= (ICompilationUnit) input;
		else if (input instanceof IMember)
			cu= ((IMember)input).getCompilationUnit();
			
		if (cu == null || !cu.exists())
			return null;
			
		
		if (cu.isWorkingCopy())
			cu= (ICompilationUnit) cu.getOriginalElement();
			
		
		IFile file= null;
		try {
			file= (IFile) cu.getUnderlyingResource();
		} catch (JavaModelException ex) {
			JavaPlugin.log(ex);
		}
		
		if (file.exists())
			return file;