diff --git a/ChangeLog b/ChangeLog
index b55745a0..9c0f6df7 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,6 +1,7 @@
 19/4/12 started 7.28.6
 - better resolution unit handling in deprecated im_vips2tiff()
 - use TIFF_CFLAGS output from pkg-config (thanks Jay)
+- much faster vips_argument_map()
 
 19/4/12 started 7.28.5
 - ifthenelse blend mode was broken
diff --git a/libvips/include/vips/object.h b/libvips/include/vips/object.h
index 42336dec..7590ac7f 100644
--- a/libvips/include/vips/object.h
+++ b/libvips/include/vips/object.h
@@ -328,15 +328,8 @@ int vips_object_get_argument_priority( VipsObject *object, const char *name );
 		VipsArgumentInstance *ARG_INSTANCE = \
 			vips__argument_get_instance( argument_class, \
 			VIPS_OBJECT( OBJECT ) ); \
-		\
-		/* We have many props on the arg table ... filter out the \
-		 * ones for this class. \
-		 */ \
-		if( g_object_class_find_property( \
-			G_OBJECT_CLASS( object_class ), \
-			g_param_spec_get_name( PSPEC ) ) == PSPEC ) {
 
-#define VIPS_ARGUMENT_FOR_ALL_END } } }
+#define VIPS_ARGUMENT_FOR_ALL_END } }
 
 /* And some macros to collect args from a va list. 
  */
@@ -494,13 +487,22 @@ struct _VipsObjectClass {
 	 */
 	const char *description;
 
-	/* Table of arguments for this class and any derived classes. Order
-	 * is important, so keep a traverse list too. We can't rely on the
-	 * ordering given by g_object_class_list_properties() since it comes
-	 * from a hash :-(
+	/* Hash from pspec to VipsArgumentClass.
+	 *
+	 * This records the VipsArgumentClass for every pspec used in 
+	 * VipsObject and any subclass (ie. everywhere), so it's huge. Don't
+	 * loop over this hash! Fine for lookups though.
 	 */
 	VipsArgumentTable *argument_table;
+
+	/* A sorted (by priority) list of the VipsArgumentClass for this class 
+	 * and any superclasses. This is small and specific to this class.
+	 *
+	 * Use the stored GType to work out when to restart the list for a
+	 * subclass.
+	 */
 	GSList *argument_table_traverse;
+	GType argument_table_traverse_gtype;
 };
 
 gboolean vips_value_is_null( GParamSpec *psoec, const GValue *value );
diff --git a/libvips/iofuncs/object.c b/libvips/iofuncs/object.c
index c6513932..53e7c879 100644
--- a/libvips/iofuncs/object.c
+++ b/libvips/iofuncs/object.c
@@ -355,9 +355,11 @@ vips_argument_map( VipsObject *object,
 	g_object_ref( object ); 
 
 	VIPS_ARGUMENT_FOR_ALL( object, 
-		pspec, argument_class, argument_instance ) {
+		pspec, argument_class, argument_instance ) { 
 		void *result;
 
+		/* argument_instance should not be NULL.
+		 */
 		g_assert( argument_instance );
 
 		if( (result = fn( object, pspec,
@@ -388,19 +390,12 @@ vips_argument_class_map( VipsObjectClass *object_class,
 			(VipsArgumentClass *) p->data; 
 		VipsArgument *argument = (VipsArgument *) arg_class; 
 		GParamSpec *pspec = argument->pspec; 
-		
-		/* We have many props on the arg table ... filter out the 
-		 * ones for this class. 
-		 */ 
-		if( g_object_class_find_property( 
-			G_OBJECT_CLASS( object_class ), 
-			g_param_spec_get_name( pspec ) ) == pspec ) {
-			void *result;
 
-			if( (result = 
-				fn( object_class, pspec, arg_class, a, b )) )
-				return( result );
-		}
+		void *result;
+
+		if( (result = 
+			fn( object_class, pspec, arg_class, a, b )) )
+			return( result );
 	}
 
 	return( NULL );
@@ -749,15 +744,11 @@ vips_object_dispose( GObject *gobject )
 	/* Our subclasses should have already called this. Run it again, just
 	 * in case.
 	 */
-	if( !object->preclose ) {
-#ifdef VIPS_DEBUG
-		printf( "vips_object_dispose: no vips_object_preclose() " );
-		vips_object_print_name( VIPS_OBJECT( gobject ) );
-		printf( "\n" );
-#endif /*VIPS_DEBUG*/
-
-		vips_object_preclose( object );
-	}
+#ifdef DEBUG
+	if( !object->preclose ) 
+		printf( "vips_object_dispose: pre-close missing!\n" );
+#endif /*DEBUG*/
+	vips_object_preclose( object );
 
 	/* Clear all our arguments: they may be holding resources we should 
 	 * drop.
@@ -1377,7 +1368,8 @@ vips_object_class_install_argument( VipsObjectClass *object_class,
 	VipsArgumentClass *argument_class = g_new( VipsArgumentClass, 1 );
 
 #ifdef DEBUG
-	printf( "vips_object_class_install_argument: %s %s\n", 
+	printf( "vips_object_class_install_argument: %p %s %s\n", 
+		object_class,
 		g_type_name( G_TYPE_FROM_CLASS( object_class ) ),
 		g_param_spec_get_name( pspec ) );
 #endif /*DEBUG*/
@@ -1401,10 +1393,46 @@ vips_object_class_install_argument( VipsObjectClass *object_class,
 
 	vips_argument_table_replace( object_class->argument_table,
 		(VipsArgument *) argument_class );
+
+	/* If this is the first argument for a new subclass, we need to clone
+	 * the traverse list we inherit.
+	 */
+	if( object_class->argument_table_traverse_gtype != 
+		G_TYPE_FROM_CLASS( object_class ) ) {
+#ifdef DEBUG
+		printf( "vips_object_class_install_argument: "
+			"cloning traverse\n" ); 
+#endif /*DEBUG*/
+
+		object_class->argument_table_traverse = 
+			g_slist_copy( object_class->argument_table_traverse );
+		object_class->argument_table_traverse_gtype = 
+			G_TYPE_FROM_CLASS( object_class );
+	}
+
 	object_class->argument_table_traverse = g_slist_prepend(
 		object_class->argument_table_traverse, argument_class );
 	object_class->argument_table_traverse = g_slist_sort(
 		object_class->argument_table_traverse, traverse_sort );
+
+#ifdef DEBUG
+{
+	GSList *p;
+
+	printf( "%d items on traverse %p\n", 
+		g_slist_length( object_class->argument_table_traverse ),
+		&object_class->argument_table_traverse );
+	for( p = object_class->argument_table_traverse; p; p = p->next ) {
+		VipsArgumentClass *argument_class = 
+			(VipsArgumentClass *) p->data;
+
+		printf( "\t%p %s\n", 
+			argument_class, 
+			g_param_spec_get_name( 
+				((VipsArgument *) argument_class)->pspec ) );
+	}
+}
+#endif /*DEBUG*/
 }
 
 /* Set a named arg from a string.