Kartograph for JSON generation is fast.

json_generation_benches.rb

require 'benchmark/ips'
require 'active_support/core_ext'
require 'jbuilder'
require 'kartograph'
require 'active_model_serializers'
require 'pry'

User = Struct.new(:id, :name, :email) do
  def read_attribute_for_serialization(attribute)
    send(attribute)
  end
end

user = User.new 1, 'Bobby', 'rross@digitalocean.com'
@users = Array.new(20, user)
@user = user

class UserMapping
  include Kartograph::DSL

  kartograph do
    root_key singular: 'user', plural: 'users', scopes: [:read]

    property :id, scopes: :read
    property :name, scopes: :read
    property :email, scopes: :read
  end
end

class UserSerializer < ActiveModel::Serializer
  self.root = 'users'
  attributes :id, :name, :email
end


Benchmark.ips do |benchmark|
  benchmark.report "Jbuilder (collection)" do
    Jbuilder.encode do |json|
      json.users @users do |user|
        json.(user, :id, :name, :email)
      end
    end
  end

  benchmark.report "Kartograph (collection)" do
    UserMapping.represent_collection_for(:read, @users)
  end

  benchmark.report "AMS (collection)" do
    ActiveModel::ArraySerializer.new(@users, root: 'users', each_serializer: UserSerializer).to_json
  end

  benchmark.report "Jbuilder (singular)" do
    Jbuilder.encode do |json|
      json.user do
        json.id    user.id
        json.name  user.name
        json.email user.email
      end
    end
  end

  benchmark.report "Kartograph (singular)" do
    UserMapping.representation_for(:read, user)
  end

  benchmark.report "AMS (singular)" do
    UserSerializer.new(user, root: 'user').to_json
  end
end

results.txt

Jbuilder (collection)   249 i/100ms
Kartograph (collection) 695 i/100ms
    AMS (collection)    224 i/100ms
 Jbuilder (singular)    2390 i/100ms
Kartograph (singular)   6502 i/100ms
      AMS (singular)    2093 i/100ms
-------------------------------------------------
Jbuilder (collection)   2520.5 (±2.8%) i/s -      12699 in   5.042246s
Kartograph (collection) 7126.3 (±3.3%) i/s -      36140 in   5.076994s
    AMS (collection)    2254.8 (±2.8%) i/s -      11424 in   5.070518s
 Jbuilder (singular)    25841.8 (±3.8%) i/s -     129060 in   5.001319s
Kartograph (singular)   79032.2 (±3.5%) i/s -     396622 in   5.024724s
      AMS (singular)    23186.9 (±3.4%) i/s -     117208 in   5.060806s