diff --git a/src/OT/Layout/GSUB/SingleSubstFormat1.hh b/src/OT/Layout/GSUB/SingleSubstFormat1.hh
index cef19cf91..5c40813dc 100644
--- a/src/OT/Layout/GSUB/SingleSubstFormat1.hh
+++ b/src/OT/Layout/GSUB/SingleSubstFormat1.hh
@@ -42,32 +42,7 @@ struct SingleSubstFormat1_3
     hb_codepoint_t d = deltaGlyphID;
     hb_codepoint_t mask = get_mask ();
 
-    if (1)
-    {
-      /* Walk the coverage and set together, to bail out when the
-       * coverage iterator runs over the end of glyphset. Otherwise,
-       * the coverage may cover many glyphs and we spend a lot of
-       * time here.  Other subtables don't have this problem because
-       * they zip coverage iterator with some array... */
-      auto c_iter = hb_iter (this+coverage);
-      auto s_iter = hb_iter (c->parent_active_glyphs ());
-      while (c_iter && s_iter)
-      {
-	hb_codepoint_t cv = *c_iter;
-	hb_codepoint_t sv = *s_iter;
-	if (cv == sv)
-	{
-	  c->output->add ((cv + d) & mask);
-	  c_iter++;
-	  s_iter++;
-	}
-	else if (cv < sv)
-	  c_iter++;
-	else
-	  s_iter++;
-      }
-    }
-    else
+    if ((this+coverage).get_population () < c->parent_active_glyphs ().get_population ())
     {
       + hb_iter (this+coverage)
       | hb_filter (c->parent_active_glyphs ())
@@ -75,6 +50,14 @@ struct SingleSubstFormat1_3
       | hb_sink (c->output)
       ;
     }
+    else
+    {
+      + hb_iter (c->parent_active_glyphs ())
+      | hb_filter (this+coverage)
+      | hb_map ([d, mask] (hb_codepoint_t g) { return (g + d) & mask; })
+      | hb_sink (c->output)
+      ;
+    }
   }
 
   void closure_lookups (hb_closure_lookups_context_t *c) const {}
@@ -136,37 +119,18 @@ struct SingleSubstFormat1_3
     hb_codepoint_t mask = get_mask ();
 
     hb_sorted_vector_t<hb_codepoint_t> intersection;
-    if (1)
-    {
-      /* Walk the coverage and set together, to bail out when the
-       * coverage iterator runs over the end of glyphset. Otherwise,
-       * the coverage may cover many glyphs and we spend a lot of
-       * time here.  Other subtables don't have this problem because
-       * they zip coverage iterator with some array... */
-      auto c_iter = hb_iter (this+coverage);
-      auto s_iter = hb_iter (glyphset);
-      while (c_iter && s_iter)
-      {
-	hb_codepoint_t cv = *c_iter;
-	hb_codepoint_t sv = *s_iter;
-	if (cv == sv)
-	{
-	  intersection.push (cv);
-	  c_iter++;
-	  s_iter++;
-	}
-	else if (cv < sv)
-	  c_iter++;
-	else
-	  s_iter++;
-      }
-    }
-    else
+    if ((this+coverage).get_population () < glyphset.get_population ())
     {
       + hb_iter (this+coverage)
       | hb_filter (glyphset)
       | hb_sink (intersection);
     }
+    else
+    {
+      + hb_iter (glyphset)
+      | hb_filter (this+coverage)
+      | hb_sink (intersection);
+    }
 
     auto it =
     + hb_iter (intersection)