https://github.com/google/incremental-dom/pull/334#issuecomment-410845374

preallocate.diff

diff --git a/src/node_data.ts b/src/node_data.ts
index 860c51e..0d712c0 100644
--- a/src/node_data.ts
+++ b/src/node_data.ts
@@ -61,8 +61,17 @@ export class NodeData {
     return !!this._attrsArr;
   }
 
-  getAttrsArr(): any[] {
-    return this._attrsArr || (this._attrsArr = []);
+  getAttrsArr(length?: number): any[] {
+    let attrs = this._attrsArr;
+    if (attrs) {
+      return attrs;
+    }
+    if (length && length > 0) {
+      attrs = new Array(length);
+    } else {
+      attrs = [];
+    }
+    return (this._attrsArr = attrs);
   }
 }
 
@@ -145,18 +154,18 @@ function recordAttributes(node: Element, data: NodeData) {
     return;
   }
 
-  const attrsArr = data.getAttrsArr();
+  const attrsArr = data.getAttrsArr(length);
 
   // Use a cached length. The attributes array is really a live NamedNodeMap,
   // which exists as a DOM "Host Object" (probably as C++ code). This makes the
   // usual constant length iteration very difficult to optimize in JITs.
-  for (let i = 0; i < length; i += 1) {
+  for (let i = 0, j = 0; i < length; i += 1, j += 2) {
     const attr = attributes[i];
     const name = attr.name;
     const value = attr.value;
 
-    attrsArr.push(name);
-    attrsArr.push(value);
+    attrsArr[j] = name;
+    attrsArr[j + 1] = value;
   }
 }
 
diff --git a/src/virtual_elements.ts b/src/virtual_elements.ts
index 221f3e4..2013dcb 100644
--- a/src/virtual_elements.ts
+++ b/src/virtual_elements.ts
@@ -133,8 +133,11 @@ function elementOpen(
     return node;
   }
 
-  const attrsArr = data.getAttrsArr();
-  const isNew = !attrsArr.length;
+  let isNew = !data.hasAttrsArr();
+  const attrsArr = data.getAttrsArr(arguments.length - ATTRIBUTES_OFFSET);
+  if (!isNew) {
+    isNew = !attrsArr.length;
+  }
 
   /*
    * Checks to see if one or more attributes have changed for a given Element.