Try caching attribute map to try to improve performance.

tools/dom/templates/html/impl/impl_Element.darttemplate

Index: tools/dom/templates/html/impl/impl_Element.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Element.darttemplate b/tools/dom/templates/html/impl/impl_Element.darttemplate
index 8f73476b88f76d38cd1c7ff3a32f1c7ef6fe064b..0bf8fd05bb76e05d8311fd479d1f2e379bb00ae8 100644
--- a/tools/dom/templates/html/impl/impl_Element.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Element.darttemplate
@@ -500,6 +500,7 @@ $(ANNOTATIONS)abstract class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
   /// This is identical to calling `new Element.tag('video')`.
   factory Element.video() => new Element.tag('video');
 
+  Map<String, String> _cachedAttributeMap;
   /**
    * All attributes on this element.
    *
@@ -510,7 +511,12 @@ $(ANNOTATIONS)abstract class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
    * (such as 'xlink:href'), additional attributes can be accessed via
    * [getNamespacedAttributes].
    */
-  Map<String, String> get attributes => new _ElementAttributeMap(this);
+  Map<String, String> get attributes {
+    if (_cachedAttributeMap == null) {
+      _cachedAttributeMap = new _ElementAttributeMap(this);
+    }
+    return _cachedAttributeMap;
+  }
 
   void set attributes(Map<String, String> value) {
     Map<String, String> attributes = this.attributes;