First Rails application - Performance & Optimization

Performance: First Rails application

MEDIUM IMPACT

This affects the initial page load speed and server response time when serving the first page of a Rails app.

Serving the home page of a new Rails app

Ruby on Rails

class HomeController < ApplicationController
  def index
    @users = User.limit(10)
  end
end

# View uses simple markup and fragment caching for partials

Limiting database records and caching view fragments reduces server processing and speeds up response.

📈 Performance Gainreduces server response time by 50-70%; improves LCP by loading main content faster

Serving the home page of a new Rails app

Ruby on Rails

class HomeController < ApplicationController
  def index
    @users = User.all
  end
end

# View renders all users with heavy partials and no caching

Loading all users and rendering many partials without caching causes slow response and delays page paint.

📉 Performance Costblocks rendering for 200-500ms depending on user count; triggers multiple database queries and view rendering delays

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Render all database records with heavy partials	High (many nodes)	Multiple reflows due to complex DOM	High paint cost	[X] Bad
Limit records and use fragment caching	Low (few nodes)	Single reflow	Low paint cost	[OK] Good

Rendering Pipeline

Rails processes the request by routing, controller action, database query, view rendering, and then sends HTML to the browser. The browser parses HTML, applies CSS, and paints the page.

→Server Processing

→Network Transfer

→Browser Parsing

→Paint

⚠️ BottleneckServer Processing due to database queries and view rendering

Core Web Vital Affected

LCP

This affects the initial page load speed and server response time when serving the first page of a Rails app.

Optimization Tips

1Limit database queries on initial page load to reduce server processing time.

2Use fragment caching in views to avoid repeated rendering of unchanged content.

3Keep views simple and avoid rendering large numbers of partials on the first page.

Performance Quiz - 3 Questions

What mainly causes slow initial page load in a new Rails app?

A Heavy database queries and complex view rendering

B Too many CSS files loaded

C Large JavaScript bundles only

D Browser caching issues

DevTools: Performance

How to check: Open DevTools > Performance tab > Record while loading the page > Look for long scripting or rendering tasks

What to look for: Long server response time, scripting delays, and large layout or paint times indicate slow Rails app rendering

Practice

1. What command do you use to create a new Rails application folder?

easy

A. rails init myapp

B. rails new myapp

C. rails create myapp

D. rails start myapp

Solution

1. Step 1: Understand Rails app creation command

   The official command to create a new Rails app is rails new followed by the app name.

2. Step 2: Match the command with options

   Only rails new myapp uses rails new myapp, which is correct syntax.

3. Final Answer:

   rails new myapp -> Option B

4. Quick Check:

   Create app folder = rails new [OK]

Hint: Remember: 'rails new' creates the app folder [OK]

Common Mistakes:

• Using 'rails start' instead of 'rails new'
• Confusing 'rails create' with 'rails new'
• Trying 'rails init' which is not a Rails command

2. Which command starts the Rails server so you can view your app in a browser?

easy

A. rails launch

B. rails start

C. rails run

D. rails server

Solution

1. Step 1: Identify the command to run the Rails server

   The correct command to start the Rails server is rails server.

2. Step 2: Compare with options

   Only rails server matches the correct command exactly.

3. Final Answer:

   rails server -> Option D

4. Quick Check:

   Start server = rails server [OK]

Hint: Use 'rails server' to launch your app locally [OK]

Common Mistakes:

• Typing 'rails start' which is invalid
• Using 'rails run' which does not start the server
• Trying 'rails launch' which is not a Rails command

3. What will happen if you run rails new blog and then cd blog followed by rails server?

medium

A. A new Rails app named 'blog' is created and the server starts to serve it locally.

B. An error occurs because 'rails server' must be run before 'rails new'.

C. The server starts but shows a blank page because no views are created yet.

D. The command fails because 'cd blog' is not a valid Rails command.

Solution

1. Step 1: Understand the sequence of commands

   rails new blog creates a new app folder named 'blog'. cd blog moves into that folder. rails server starts the server for that app.

2. Step 2: Analyze the expected behavior

   Running these commands in order creates the app and starts the server correctly, serving the default Rails welcome page.

3. Final Answer:

   A new Rails app named 'blog' is created and the server starts to serve it locally. -> Option A

4. Quick Check:

   rails new + cd + rails server = app runs [OK]

Hint: Create app, enter folder, then start server to see app [OK]

Common Mistakes:

• Thinking 'rails server' must run before app creation
• Assuming 'cd blog' is a Rails command instead of shell
• Expecting an error when running these commands in order

4. You ran rails new myapp but when you try rails server inside the folder, you get an error. What is the most likely cause?

medium

A. You must create a controller before starting the server.

B. You need to run rails start instead of rails server.

C. You forgot to run bundle install to install dependencies.

D. The folder name 'myapp' is invalid for Rails apps.

Solution

1. Step 1: Identify common setup step after creating app

   After rails new, you usually run bundle install to install gems needed by the app.

2. Step 2: Understand error cause

   If dependencies are missing, rails server will fail to start due to missing gems.

3. Final Answer:

   You forgot to run bundle install to install dependencies. -> Option C

4. Quick Check:

   Missing gems? Run bundle install first [OK]

Hint: Run 'bundle install' after 'rails new' before starting server [OK]

Common Mistakes:

• Trying 'rails start' which is invalid
• Thinking a controller is required before server start
• Believing folder name causes the error

5. You want to create a new Rails app named shop and immediately start the server. Which sequence of commands is correct?

hard

A. rails new shop -> cd shop -> rails server

B. rails create shop -> rails server

C. rails new shop -> rails server

D. cd shop -> rails new shop -> rails server

Solution

1. Step 1: Create the app folder with correct command

   Use rails new shop to create the app folder named 'shop'.

2. Step 2: Change directory into the new app folder

   Run cd shop to move into the app folder before starting the server.

3. Step 3: Start the Rails server inside the app folder

   Run rails server to launch the app locally.

4. Final Answer:

   rails new shop -> cd shop -> rails server -> Option A

5. Quick Check:

   Create app, enter folder, start server = correct order [OK]

Hint: Create app, cd into it, then run server [OK]

Common Mistakes:

• Skipping 'cd' before running server
• Using 'rails create' which is invalid
• Running server outside app folder