Spec Performance

Slow tests reduce productivity and discourage running tests frequently. This skill covers techniques for optimizing RSpec test suite performance.

Profiling Slow Tests

Built-in Profiler

# Show 10 slowest examples
rspec --profile 10

# In spec_helper.rb for always-on profiling
RSpec.configure do |config|
  config.profile_examples = 10
end

Detailed Timing

# spec/support/timing.rb
RSpec.configure do |config|
  config.around(:each) do |example|
    start = Time.now
    example.run
    elapsed = Time.now - start
    puts "#{example.full_description}: #{elapsed.round(2)}s" if elapsed > 1
  end
end

Database Optimization

Prefer build Over create

# Slow - hits database
let(:user) { create(:user) }

# Fast - in memory only
let(:user) { build(:user) }

# Fastest - stubbed, no DB
let(:user) { build_stubbed(:user) }

When to Use Each Strategy

Strategy	Use When
build	Testing validations, object behavior
build_stubbed	Need ID, testing presentation
create	Testing DB queries, associations, callbacks

Minimize Database Writes

# Slow - creates 3 users
it "lists users" do
  create(:user)
  create(:user)
  create(:user)
  expect(User.count).to eq(3)
end

# Better - single batch insert
it "lists users" do
  User.insert_all([
    { email: "a@test.com" },
    { email: "b@test.com" },
    { email: "c@test.com" }
  ])
  expect(User.count).to eq(3)
end

Use before(:all) Carefully

# Creates user once for all examples
before(:all) do
  @user = create(:user)
end

after(:all) do
  @user.destroy
end

# Warning: shared state between examples
# Use only for read-only data

let vs let!

let - Lazy Evaluation

let(:user) { create(:user) }

it "does something" do
  # User created here, when first accessed
  user.name
end

it "does something else" do
  # User not created if not referenced
  expect(true).to be true
end

let! - Eager Evaluation

let!(:user) { create(:user) }

it "has a user in database" do
  # User already created before this runs
  expect(User.count).to eq(1)
end

When to Use let!

# Use let! when:
# 1. Callback side effects needed
let!(:user) { create(:user) }  # Triggers after_create callbacks

# 2. Database state must exist before test
let!(:existing_user) { create(:user, email: "taken@test.com") }

it "validates uniqueness" do
  new_user = build(:user, email: "taken@test.com")
  expect(new_user).not_to be_valid
end

Avoid Unnecessary let!

# Bad - always creates even when not needed
let!(:user) { create(:user) }
let!(:post) { create(:post, user: user) }
let!(:comment) { create(:comment, post: post) }

# Good - only create what's needed per test
let(:user) { create(:user) }

context "with posts" do
  let(:post) { create(:post, user: user) }

  it "lists posts" do
    post  # Triggers creation
    expect(user.posts).to include(post)
  end
end

Parallel Testing

parallel_tests Gem

# Gemfile
gem "parallel_tests", group: [:development, :test]

# Setup
rake parallel:setup
rake parallel:create
rake parallel:migrate

# Run tests
rake parallel:spec
# or
parallel_rspec spec/

Database Configuration

# config/database.yml
test:
  database: myapp_test<%= ENV['TEST_ENV_NUMBER'] %>

Avoiding Parallelization Issues

# Use unique data per process
let(:email) { "user#{Process.pid}@test.com" }

# Avoid shared files
let(:file_path) { Rails.root.join("tmp/test_#{Process.pid}.txt") }

Mocking and Stubbing for Speed

Stub External Services

# Slow - real HTTP call
it "fetches data" do
  result = ExternalApi.fetch(id: 1)
  expect(result).to be_present
end

# Fast - stubbed response
it "fetches data" do
  allow(ExternalApi).to receive(:fetch).and_return({ data: "test" })
  result = ExternalApi.fetch(id: 1)
  expect(result).to eq({ data: "test" })
end

Use WebMock for HTTP

# Gemfile
gem "webmock", group: :test

# spec/spec_helper.rb
require "webmock/rspec"
WebMock.disable_net_connect!(allow_localhost: true)

# In specs
stub_request(:get, "https://api.example.com/users")
  .to_return(status: 200, body: { users: [] }.to_json)

Stub Time-Consuming Methods

# Slow - actual file processing
it "processes file" do
  result = FileProcessor.process(large_file)
  expect(result).to be_present
end

# Fast - stub the slow method
it "processes file" do
  allow(FileProcessor).to receive(:process).and_return({ success: true })
  result = FileProcessor.process(large_file)
  expect(result).to eq({ success: true })
end

Test Data Optimization

Use build_stubbed for Speed

# build_stubbed creates objects with:
# - Fake IDs (but not nil)
# - Fake timestamps
# - No database writes

user = build_stubbed(:user)
user.id        # => 1001 (fake but present)
user.persisted? # => true (pretends to be saved)

Minimal Factory Attributes

# Slow - lots of associations
factory :order do
  user
  shipping_address
  billing_address
  coupon
  association :items, count: 5
end

# Fast - minimal required attributes
factory :order do
  status { "pending" }
  total { 100 }

  trait :with_user do
    user
  end
end

Avoid N+1 in Test Setup

# Slow - N+1 queries in setup
let(:users) { create_list(:user, 10) }

before do
  users.each { |u| create(:post, user: u) }
end

# Better - batch operations
before do
  users = create_list(:user, 10)
  Post.insert_all(users.map { |u| { user_id: u.id, title: "Post" } })
end

CI Optimization

Fail Fast

# spec/spec_helper.rb
RSpec.configure do |config|
  config.fail_fast = ENV["CI"].present?
end

Bisect for Flaky Tests

# Find minimal set that reproduces failure
rspec --bisect

Example Status Persistence

# spec/spec_helper.rb
RSpec.configure do |config|
  # Re-run only failed specs first
  config.example_status_persistence_file_path = "spec/examples.txt"
end

Performance Checklist

Quick Wins

• Use build instead of create when possible
• Use build_stubbed for presentation tests
• Stub external HTTP calls
• Avoid let! when let works

Medium Effort

• Set up parallel testing
• Profile and fix slowest specs
• Minimize factory associations
• Use before(:all) for read-only setup

Advanced

• Database cleaner strategy optimization
• Shared database connections for system specs
• Spring preloader for development
• CI caching for gems and assets

Benchmarking Helpers

# spec/support/benchmark_helper.rb
module BenchmarkHelper
  def measure(label = "Block", &block)
    start = Process.clock_gettime(Process::CLOCK_MONOTONIC)
    result = block.call
    elapsed = Process.clock_gettime(Process::CLOCK_MONOTONIC) - start
    puts "#{label}: #{(elapsed * 1000).round(2)}ms"
    result
  end
end

# Usage in specs
include BenchmarkHelper

it "performs quickly" do
  measure("User creation") { create(:user) }
  measure("User build") { build(:user) }
end

Additional Resources

Reference Files

• references/parallel-testing.md - Parallel test configuration
• references/ci-optimization.md - CI-specific optimizations

Example Files

• examples/fast_spec.rb - Optimized spec patterns
• examples/slow_spec.rb - Anti-patterns to avoid