docs: add perf todo notes from angular#8515

Author: IgorMinar

src/ng/compile.js

@@ -2195,6 +2195,8 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
       if (parent) {
         parent.replaceChild(newNode, firstElementToRemove);
       }
+
+      // TODO(perf): what's this document fragment for? is it needed? can we at least reuse it?
       var fragment = document.createDocumentFragment();
       fragment.appendChild(firstElementToRemove);
 

src/ng/directive/input.js

@@ -2131,6 +2131,7 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
     }
 
     // if scope model value and ngModel value are out of sync
+    // TODO(perf): why not move this to the action fn?
     if (ctrl.$modelValue !== modelValue &&
         (isUndefined(ctrl.$$invalidModelValue) || ctrl.$$invalidModelValue != modelValue)) {
 

src/ng/directive/ngRepeat.js

@@ -419,6 +419,8 @@ var ngRepeatDirective = ['$parse', '$animate', function($parse, $animate) {
                 // http://jsperf.com/clone-vs-createcomment
                 var endNode = ngRepeatEndComment.cloneNode(false);
                 clone[clone.length++] = endNode;
+
+                // TODO(perf): support naked previousNode in `enter` to avoid creation of jqLite wrapper?
                 $animate.enter(clone, null, jqLite(previousNode));
                 previousNode = endNode;
                 // Note: We only need the first/last node of the cloned nodes.