diff --git a/bundles/org.eclipse.swt/Eclipse SWT PI/gtk/library/os.c b/bundles/org.eclipse.swt/Eclipse SWT PI/gtk/library/os.c
index 58a716c597e..2d4ac759a1e 100644
--- a/bundles/org.eclipse.swt/Eclipse SWT PI/gtk/library/os.c	
+++ b/bundles/org.eclipse.swt/Eclipse SWT PI/gtk/library/os.c	
@@ -4186,6 +4186,16 @@ JNIEXPORT void JNICALL GTK_NATIVE(gtk_1combo_1box_1set_1active)
 }
 #endif
 
+#ifndef NO_gtk_1combo_1box_1set_1model
+JNIEXPORT void JNICALL GTK_NATIVE(gtk_1combo_1box_1set_1model)
+	(JNIEnv *env, jclass that, jlong arg0, jlong arg1)
+{
+	GTK_NATIVE_ENTER(env, that, gtk_1combo_1box_1set_1model_FUNC);
+	gtk_combo_box_set_model((GtkComboBox *)arg0, (GtkTreeModel *)arg1);
+	GTK_NATIVE_EXIT(env, that, gtk_1combo_1box_1set_1model_FUNC);
+}
+#endif
+
 #ifndef NO_gtk_1combo_1box_1text_1insert
 JNIEXPORT void JNICALL GTK_NATIVE(gtk_1combo_1box_1text_1insert)
 	(JNIEnv *env, jclass that, jlong arg0, jint arg1, jbyteArray arg2, jbyteArray arg3)
diff --git a/bundles/org.eclipse.swt/Eclipse SWT PI/gtk/library/os_stats.h b/bundles/org.eclipse.swt/Eclipse SWT PI/gtk/library/os_stats.h
index 442262a9d75..95a9509c9c0 100644
--- a/bundles/org.eclipse.swt/Eclipse SWT PI/gtk/library/os_stats.h	
+++ b/bundles/org.eclipse.swt/Eclipse SWT PI/gtk/library/os_stats.h	
@@ -328,6 +328,7 @@ typedef enum {
 	gtk_1combo_1box_1popdown_FUNC,
 	gtk_1combo_1box_1popup_FUNC,
 	gtk_1combo_1box_1set_1active_FUNC,
+	gtk_1combo_1box_1set_1model_FUNC,
 	gtk_1combo_1box_1text_1insert_FUNC,
 	gtk_1combo_1box_1text_1new_FUNC,
 	gtk_1combo_1box_1text_1new_1with_1entry_FUNC,
diff --git a/bundles/org.eclipse.swt/Eclipse SWT PI/gtk/org/eclipse/swt/internal/gtk/GTK.java b/bundles/org.eclipse.swt/Eclipse SWT PI/gtk/org/eclipse/swt/internal/gtk/GTK.java
index 9a6c2cb9cf8..4d7f140abc1 100644
--- a/bundles/org.eclipse.swt/Eclipse SWT PI/gtk/org/eclipse/swt/internal/gtk/GTK.java	
+++ b/bundles/org.eclipse.swt/Eclipse SWT PI/gtk/org/eclipse/swt/internal/gtk/GTK.java	
@@ -528,7 +528,6 @@ public class GTK extends OS {
 	/**
 	 * @param combo_box cast=(GtkComboBoxText *)
 	 */
-	/* Do not call directly. Call Combo.gtk_combo_box_text_remove_all(..) instead). */
 	public static final native void gtk_combo_box_text_remove_all(long combo_box);
 	/**
 	* @param combo_box cast=(GtkComboBox *)
@@ -540,6 +539,11 @@ public class GTK extends OS {
 	public static final native long gtk_combo_box_get_model(long combo_box);
 	/**
 	* @param combo_box cast=(GtkComboBox *)
+	* @param model cast=(GtkTreeModel *)
+	*/
+	public static final native void gtk_combo_box_set_model(long combo_box, long model);
+	/**
+	* @param combo_box cast=(GtkComboBox *)
 	* @param index cast=(gint)
 	*/
 	public static final native void gtk_combo_box_set_active(long combo_box, int index);
diff --git a/bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/widgets/Combo.java b/bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/widgets/Combo.java
index d58fed8720e..77866968a01 100644
--- a/bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/widgets/Combo.java	
+++ b/bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/widgets/Combo.java	
@@ -2067,12 +2067,45 @@ public void remove (int start, int end) {
 	items = newItems;
 	int index = GTK.gtk_combo_box_get_active (handle);
 	if (start <= index && index <= end) clearText();
+	/*
+	 * Detaching the model below resets the active item to -1. If the previously
+	 * selected item is outside the removed range it must be restored afterwards,
+	 * adjusted for the rows removed before it.
+	 */
+	int newIndex = -1;
+	if (index != -1 && !(start <= index && index <= end)) {
+		newIndex = index > end ? index - (end - start + 1) : index;
+	}
 
-	gtk_combo_box_toggle_wrap(false);
-	for (int i = end; i >= start; i--) {
-		if (handle != 0) GTK.gtk_combo_box_text_remove(handle, i);
+	long model = handle != 0 ? GTK.gtk_combo_box_get_model (handle) : 0;
+	if (model != 0) {
+		/*
+		 * Bug 506: Removing a range of items one-by-one is slow because the
+		 * GtkComboBox reacts to every single row deletion. Detach the model,
+		 * remove the rows directly from the GtkListStore (from the end of the
+		 * range downwards so indices stay stable), and re-attach it so the
+		 * widget only updates once.
+		 */
+		OS.g_object_ref (model);
+		gtk_combo_box_toggle_wrap (false);
+		GTK.gtk_combo_box_set_model (handle, 0);
+		long iter = OS.g_malloc (GTK.GtkTreeIter_sizeof ());
+		for (int i = end; i >= start; i--) {
+			if (GTK.gtk_tree_model_iter_nth_child (model, iter, 0, i)) {
+				GTK.gtk_list_store_remove (model, iter);
+			}
+		}
+		OS.g_free (iter);
+		GTK.gtk_combo_box_set_model (handle, model);
+		if (newIndex != -1) {
+			// Restore the selection lost by detaching the model, without firing a spurious Modify event.
+			OS.g_signal_handlers_block_matched (handle, OS.G_SIGNAL_MATCH_DATA, 0, 0, 0, 0, CHANGED);
+			GTK.gtk_combo_box_set_active (handle, newIndex);
+			OS.g_signal_handlers_unblock_matched (handle, OS.G_SIGNAL_MATCH_DATA, 0, 0, 0, 0, CHANGED);
+		}
+		OS.g_object_unref (model);
+		gtk_combo_box_toggle_wrap (true);
 	}
-	gtk_combo_box_toggle_wrap(true);
 }
 
 /**
@@ -2112,7 +2145,23 @@ public void removeAll () {
 
 	items = new String[0];
 	clearText();
-	gtk_combo_box_text_remove_all();
+
+	long model = handle != 0 ? GTK.gtk_combo_box_get_model (handle) : 0;
+	if (model != 0) {
+		/*
+		 * Bug 506: Removing a large number of combo items is slow because the
+		 * GtkComboBox reacts to every single row deletion. Detach the model
+		 * from the combo box, clear the GtkListStore in one step, and re-attach
+		 * it so the widget only updates once.
+		 */
+		OS.g_object_ref (model);
+		gtk_combo_box_toggle_wrap (false);
+		GTK.gtk_combo_box_set_model (handle, 0);
+		GTK.gtk_list_store_clear (model);
+		GTK.gtk_combo_box_set_model (handle, model);
+		OS.g_object_unref (model);
+		gtk_combo_box_toggle_wrap (true);
+	}
 }
 
 /**
@@ -2401,20 +2450,50 @@ public void setItems (String... items) {
 	System.arraycopy (items, 0, this.items, 0, items.length);
 	clearText ();
 
-	gtk_combo_box_text_remove_all();
-	for (int i = 0; i < items.length; i++) {
-		String string = items [i];
-		gtk_combo_box_insert(string, i);
-		if ((style & SWT.RIGHT_TO_LEFT) != 0 && popupHandle != 0) {
-			GTK3.gtk_container_forall (popupHandle, display.setDirectionProc, GTK.GTK_TEXT_DIR_RTL);
+	long model = handle != 0 ? GTK.gtk_combo_box_get_model (handle) : 0;
+	if (model != 0) {
+		/*
+		 * Bug 506: Setting a large number of combo items is slow because the
+		 * GtkComboBox recomputes its popup/cell-view layout on every single
+		 * model change, resulting in O(n^2) behavior. The fix is to temporarily
+		 * detach the model from the combo box, populate the GtkListStore
+		 * directly, and then re-attach the model. This way the widget reacts
+		 * only once instead of once per inserted item.
+		 */
+		OS.g_object_ref (model);
+		gtk_combo_box_toggle_wrap (false);
+		GTK.gtk_combo_box_set_model (handle, 0);
+		GTK.gtk_list_store_clear (model);
+		long iter = OS.g_malloc (GTK.GtkTreeIter_sizeof ());
+		for (int i = 0; i < items.length; i++) {
+			gtk_list_store_insert (model, iter, items [i], i);
 		}
+		OS.g_free (iter);
+		GTK.gtk_combo_box_set_model (handle, model);
+		OS.g_object_unref (model);
+		gtk_combo_box_toggle_wrap (true);
+	}
+
+	if ((style & SWT.RIGHT_TO_LEFT) != 0 && popupHandle != 0) {
+		GTK3.gtk_container_forall (popupHandle, display.setDirectionProc, GTK.GTK_TEXT_DIR_RTL);
 	}
 }
 
-private void gtk_combo_box_text_remove_all() {
-	gtk_combo_box_toggle_wrap(false);
-	if (handle != 0) GTK.gtk_combo_box_text_remove_all(handle);
-	gtk_combo_box_toggle_wrap(true);
+/**
+ * Inserts an item directly into the combo's underlying GtkListStore, bypassing
+ * the GtkComboBoxText convenience functions (which require the model to be
+ * attached to the combo box). This is used during bulk operations while the
+ * model is temporarily detached, see {@link #setItems}.
+ * <p>
+ * The text of a GtkComboBoxText is stored in column 0 of its model, matching
+ * the cell renderer attribute configured in {@code createHandle}. The caller
+ * passes a single reusable {@code iter} to avoid per-item allocation overhead.
+ * </p>
+ */
+private void gtk_list_store_insert (long model, long iter, String string, int index) {
+	byte[] buffer = Converter.wcsToMbcs (string, true);
+	GTK.gtk_list_store_insert (model, iter, index);
+	GTK.gtk_list_store_set (model, iter, 0, buffer, -1);
 }
 
 /**