Try caching attribute map to try to improve performance.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of $LIBRARYNAME;
 
 class _ChildrenElementList extends ListBase<Element> {
   // Raw Element.
   final Element _element;
   final HtmlCollection _childElements;
@@ skipping 482 matching lines @@
   /// Creates a new `<ul>` element.
   ///
   /// This is identical to calling `new Element.tag('ul')`.
   factory Element.ul() => new Element.tag('ul');
 
   /// Creates a new `<video>` element.
   ///
   /// 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.
    *
    * Any modifications to the attribute map will automatically be applied to
    * this element.
    *
    * This only includes attributes which are not in a namespace
    * (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;
     attributes.clear();
     for (String key in value.keys) {
       attributes[key] = value[key];
     }
   }
 
   /**
@@ skipping 829 matching lines @@
   const ScrollAlignment._internal(this._value);
   toString() => 'ScrollAlignment.$_value';
 
   /// Attempt to align the element to the top of the scrollable area.
   static const TOP = const ScrollAlignment._internal('TOP');
   /// Attempt to center the element in the scrollable area.
   static const CENTER = const ScrollAlignment._internal('CENTER');
   /// Attempt to align the element to the bottom of the scrollable area.
   static const BOTTOM = const ScrollAlignment._internal('BOTTOM');
 }