Remember last non-null following sibling.

This way, we don’t rescan the same null elements.

d3.js

@@ -924,9 +924,12 @@ d3 = function() {
     return d3_selectionPrototype.insert.call(this, name, before);
   };
   function d3_selection_enterInsertBefore(enter) {
+    var i0, j0;
     return function(d, i, j) {
       var group = enter[j].update, n = group.length, node;
-      while (++i < n && !(node = group[i])) ;
+      if (j != j0) j0 = j, i0 = 0;
+      if (i >= i0) i0 = i + 1;
+      while (!(node = group[i0]) && ++i0 < n) ;
       return node;
     };
   }

src/selection/enter-insert.js

@@ -7,11 +7,14 @@ d3_selection_enterPrototype.insert = function(name, before) {
 };
 
 function d3_selection_enterInsertBefore(enter) {
+  var i0, j0;
   return function(d, i, j) {
     var group = enter[j].update,
         n = group.length,
         node;
-    while (++i < n && !(node = group[i]));
+    if (j != j0) j0 = j, i0 = 0;
+    if (i >= i0) i0 = i + 1;
+    while (!(node = group[i0]) && ++i0 < n);
     return node;
   };
 }

test/selection/insert-test.js

@@ -120,7 +120,7 @@ suite.addBatch({
         return body;
       },
       "inserts before the following updating sibling": function(body) {
-        var data = ["peach", "apple", "banana", "orange", "apricot"];
+        var data = ["peach", "apple", "apple2", "apple3", "banana", "orange", "apricot"];
         body.selectAll("div").data(data, String).enter().insert("div");
         assert.deepEqual(body.selectAll("div").data(), data);
       }