Add benchmarks for adapters

Gemfile

@@ -45,4 +45,5 @@ end
 
 group :development, :test do
   gem 'rubocop', '~> 0.34.0', require: false
+  gem 'benchmark-ips'
 end

bench/adapters.rb

@@ -0,0 +1,52 @@
+#!/usr/bin/env ruby
+
+require 'rubygems'
+require 'bundler'
+Bundler.setup
+
+require 'active_model_serializers'
+require 'active_support/all'
+require 'benchmark/ips'
+require_relative './../test/fixtures/poro'
+
+def bench_adapters(resource)
+  serializer = PostWithCustomKeysSerializer.new(resource)
+
+  Benchmark.ips do |x|
+    ActiveModel::Serializer::Adapter.adapter_map.each do |adapter_name, adapter_class|
+      x.report(adapter_name) { adapter_class.new(serializer).to_json } unless adapter_name == 'null'
+    end
+
+    x.compare!
+  end
+end
+
+def bench_with_associations
+  post = Post.new(id: 1, title: 'New Post', body: 'Body')
+  author = Author.new(id: 1, name: 'Ian K')
+  first_comment = Comment.new(id: 1, body: 'ZOMG A COMMENT')
+  second_comment = Comment.new(id: 2, body: 'ZOMG ANOTHER COMMENT')
+  post.comments = [first_comment, second_comment]
+  first_comment.post = post
+  second_comment.post = post
+  post.author = author
+  blog = Blog.new(id: 1, name: 'My Blog!!')
+  post.blog = blog
+
+  bench_adapters(post)
+end
+
+def bench_without_associations
+  post = Post.new(id: 1, title: 'New Post', body: 'Body')
+  bench_adapters(post)
+end
+
+puts '=== Benchmarking resource with associations ======'
+puts
+bench_with_associations
+puts
+
+puts '=== Benchmarking resource without associations ==='
+puts
+bench_without_associations
+puts