diff --git a/gtk2_ardour/port_group.cc b/gtk2_ardour/port_group.cc
index f64b1031ef..149baf3046 100644
--- a/gtk2_ardour/port_group.cc
+++ b/gtk2_ardour/port_group.cc
@@ -159,7 +159,7 @@ PortGroup::total_channels () const
 /** PortGroupList constructor.
  */
 PortGroupList::PortGroupList ()
-	: _type (DataType::AUDIO), _signals_suspended (false), _pending_change (false)
+	: _type (DataType::AUDIO), _signals_suspended (false), _pending_change (false), _bundles_dirty (true)
 {
 	
 }
@@ -319,6 +319,7 @@ PortGroupList::gather (ARDOUR::Session& session, bool inputs)
 	add_group (other);
 
 	emit_changed ();
+	_bundles_dirty = true;
 }
 
 boost::shared_ptr<Bundle>
@@ -402,18 +403,23 @@ PortGroupList::clear ()
 	_bundle_changed_connections.clear ();
 
 	emit_changed ();
+	_bundles_dirty = true;
 }
 
 
 PortGroup::BundleList const &
 PortGroupList::bundles () const
 {
-	_bundles.clear ();
-
-	for (PortGroupList::List::const_iterator i = begin (); i != end (); ++i) {
-		if ((*i)->visible()) {
-			std::copy ((*i)->bundles().begin(), (*i)->bundles().end(), std::back_inserter (_bundles));
+	if (_bundles_dirty) {
+		_bundles.clear ();
+		
+		for (PortGroupList::List::const_iterator i = begin (); i != end (); ++i) {
+			if ((*i)->visible()) {
+				std::copy ((*i)->bundles().begin(), (*i)->bundles().end(), std::back_inserter (_bundles));
+			}
 		}
+
+		_bundles_dirty = false;
 	}
 
 	return _bundles;
@@ -446,6 +452,7 @@ PortGroupList::add_group (boost::shared_ptr<PortGroup> g)
 		);
 
 	emit_changed ();
+	_bundles_dirty = true;
 }
 
 void
@@ -456,6 +463,7 @@ PortGroupList::remove_bundle (boost::shared_ptr<Bundle> b)
 	}
 
 	emit_changed ();
+	_bundles_dirty = true;
 }
 
 void
diff --git a/gtk2_ardour/port_group.h b/gtk2_ardour/port_group.h
index f3f84a23ad..21dc9445cc 100644
--- a/gtk2_ardour/port_group.h
+++ b/gtk2_ardour/port_group.h
@@ -134,6 +134,7 @@ class PortGroupList : public sigc::trackable
 
 	ARDOUR::DataType _type;
 	mutable PortGroup::BundleList _bundles;
+	mutable bool _bundles_dirty;
 	List _groups;
 	std::vector<sigc::connection> _bundle_changed_connections;
 	bool _signals_suspended;
diff --git a/gtk2_ardour/port_matrix_column_labels.cc b/gtk2_ardour/port_matrix_column_labels.cc
index e630dec4c0..92affb1f31 100644
--- a/gtk2_ardour/port_matrix_column_labels.cc
+++ b/gtk2_ardour/port_matrix_column_labels.cc
@@ -413,8 +413,9 @@ PortMatrixColumnLabels::channel_x (ARDOUR::BundleChannel const &bc) const
 {
 	uint32_t n = 0;
 
-	PortGroup::BundleList::const_iterator i = _matrix->columns()->bundles().begin();
-	while (i != _matrix->columns()->bundles().end() && i->bundle != bc.bundle) {
+	PortGroup::BundleList const & b = _matrix->columns()->bundles ();
+	PortGroup::BundleList::const_iterator i = b.begin();
+	while (i != b.end() && i->bundle != bc.bundle) {
 		if (_matrix->show_only_bundles()) {
 			n += 1;
 		} else {
@@ -426,7 +427,7 @@ PortMatrixColumnLabels::channel_x (ARDOUR::BundleChannel const &bc) const
 	if (!_matrix->show_only_bundles()) {
 		n += bc.channel;
 	}
-	
+
 	return n * column_width();
 }
 
diff --git a/gtk2_ardour/port_matrix_labels.cc b/gtk2_ardour/port_matrix_labels.cc
index 8b34b71ffb..d29a9244df 100644
--- a/gtk2_ardour/port_matrix_labels.cc
+++ b/gtk2_ardour/port_matrix_labels.cc
@@ -25,7 +25,6 @@ void
 PortMatrixLabels::draw_extra (cairo_t* cr)
 {
 	for (std::vector<ARDOUR::BundleChannel>::const_iterator i = _channel_highlights.begin(); i != _channel_highlights.end(); ++i) {
-
 		if (_matrix->show_only_bundles()) {
 			render_bundle_name (
 				cr,
@@ -50,7 +49,6 @@ void
 PortMatrixLabels::clear_channel_highlights ()
 {
 	for (std::vector<ARDOUR::BundleChannel>::const_iterator i = _channel_highlights.begin(); i != _channel_highlights.end(); ++i) {
-
 		queue_draw_for (*i);
 	}
 
diff --git a/gtk2_ardour/port_matrix_row_labels.cc b/gtk2_ardour/port_matrix_row_labels.cc
index ded591278e..4802d4ca33 100644
--- a/gtk2_ardour/port_matrix_row_labels.cc
+++ b/gtk2_ardour/port_matrix_row_labels.cc
@@ -316,8 +316,9 @@ PortMatrixRowLabels::channel_y (ARDOUR::BundleChannel const& bc) const
 {
 	uint32_t n = 0;
 
-	PortGroup::BundleList::const_iterator i = _matrix->rows()->bundles().begin();
-	while (i != _matrix->rows()->bundles().end() && i->bundle != bc.bundle) {
+	PortGroup::BundleList const & bundles = _matrix->rows()->bundles();
+	PortGroup::BundleList::const_iterator i = bundles.begin ();
+	while (i != bundles.end() && i->bundle != bc.bundle) {
 		if (_matrix->show_only_bundles()) {
 			n += 1;
 		} else {