Further experimentation. Was able to cut the cost of rendering 100 partials in a collection in half.

  To discuss: What are the desired semantics (if any) for layouts in a collection. There are no
  tests for it at present, and I'm not sure if it's needed at all.

  Deprecated on this branch: `object` pointing at the current object in partials. You can still
  use the partial name, or use :as to achieve the same thing. This is obviously up for discussion.

1 files changed, 40 insertions, 9 deletions

diff --git a/actionpack/lib/action_view/render/partials.rb b/actionpack/lib/action_view/render/partials.rb
index 64f08c447d..2aa3bd7db8 100644
--- a/actionpack/lib/action_view/render/partials.rb
+++ b/actionpack/lib/action_view/render/partials.rb
@@ -207,9 +207,7 @@ module ActionView
       end
 
       def render_collection
-        # Even if no template is rendered, this will ensure that the MIME type
-        # for the empty response is the same as the provided template
-        @options[:_template] = default_template = find_template
+        @options[:_template] = template = find_template
 
         return nil if collection.blank?
 
@@ -217,15 +215,48 @@ module ActionView
           spacer = find_template(@options[:spacer_template]).render(@view, @locals)
         end
 
-        segments = []
+        result = template ? collection_with_template(template) : collection_without_template
+        result.join(spacer)
+      end
+
+      def collection_with_template(template)
+        options = @options
+
+        segments, locals, as = [], @locals, options[:as]
+
+        [].tap do |segments|
+          variable_name = template.variable_name
+          counter_name  = template.counter_name
+          locals[counter_name] = -1
 
-        collection.each_with_index do |object, index|
-          template = default_template || find_template(partial_path(object))
-          @locals[template.counter_name] = index
-          segments << render_template(template, object)
+          collection.each do |object|
+            locals[counter_name] += 1
+            locals[variable_name] = object
+            locals[as] = object if as
+
+            segments << template.render(@view, locals)
+          end
         end
+      end
 
-        segments.join(spacer)
+      def collection_without_template
+        options = @options
+
+        segments, locals, as = [], @locals, options[:as]
+        index, template = -1, nil
+
+        [].tap do |segments|
+          collection.each do |object|
+            template = find_template(partial_path(object))
+            locals[template.counter_name] = (index += 1)
+            locals[template.variable_name] = object
+            locals[as] = object if as
+
+            segments << template.render(@view, locals)
+          end
+
+          @options[:_template] = template
+        end
       end
 
       def render_template(template, object = @object)