Tremendous speedup for NSMetadataQuery

In QNote I am using a NSMetadataQuery to allow the user to search for notes. Even with a notes database of a trivial size, it took ages to find notes (it is basically faster to open a terminal and issue a grep command ). And this even when
I have already restricted the searchScope to the directory with the notes in them.

Today I was experimenting with Spotlight again (is there really good documentation for it out there?).

When I add the type of the data I am expecting (plain text), the search gets real quick - exactly as expected. So the code now looks like this:

  // Predicate expression for the search string
  NSString *predicateFormat = @"kMDItemTextContent like[cd] %@";
 	
  // Predicate expression for 
  NSString *textType = @"kMDItemContentType == 'public.plain-text'";
 
  // If there is something to search for ..
  if (_searchKey != nil ) 
  {
    // Create the predicate for the key
    NSPredicate *p = [NSPredicate predicateWithFormat:predicateFormat,
 [_searchKey stringByAppendingString:@"*"]];
 
    // And one for the type
    NSPredicate * subPredicate = [NSPredicate predicateWithFormat:
	  textType];
 
    // Combine the two
    p = [NSCompoundPredicate andPredicateWithSubpredicates:
            [NSArray arrayWithObjects:p, subPredicate, nil]];
     
    // And set on the query
    [_query setPredicate:p];

This simple addition of kMDItemContentType == 'public.plain-text' provides the speedup.
It is unfortunately not clear to me why, as I don't have any other files of other types in the searchScope.

Next thing for me to investigate is not why this spinning thingy (they are called async arrows :) does not stop spinning when spotlight does no longer return results. Perhaps I need to programatically set this and can't rely on the cocoa bindings here - does anyone know?