API-only application setup in Ruby on Rails - Performance & Optimization

Performance: API-only application setup

MEDIUM IMPACT

This setup affects initial page load speed and server response time by minimizing unnecessary frontend assets and middleware.

Setting up a Rails backend to serve only JSON APIs without frontend views

Ruby on Rails

rails new myapp --api
# API-only mode disables views, asset pipeline, and reduces middleware

Smaller server response size and faster JSON API responses by removing unused frontend code.

📈 Performance Gainreduces response size by 80-90 KB, improves LCP by faster backend response

Setting up a Rails backend to serve only JSON APIs without frontend views

Ruby on Rails

rails new myapp
# default full-stack setup with views, assets, and middleware
# serves HTML views and includes asset pipeline

Includes unnecessary middleware and frontend assets that increase server response size and slow down API response.

📉 Performance Costadds 100+ KB to server response, blocks rendering with asset compilation

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Full-stack Rails app serving HTML views	Many DOM nodes generated by views	Multiple reflows due to large DOM	High paint cost from complex UI	[X] Bad
Rails API-only app serving JSON	No DOM generated by server	No reflows triggered by server	No paint cost from server response	[OK] Good

Rendering Pipeline

In API-only mode, the server skips view rendering and asset compilation, directly sending JSON responses. This reduces backend processing and network payload, speeding up the critical rendering path on the client.

→Server Processing

→Network Transfer

→Client Rendering

⚠️ BottleneckServer Processing due to view rendering and asset compilation in full-stack mode

Core Web Vital Affected

LCP

This setup affects initial page load speed and server response time by minimizing unnecessary frontend assets and middleware.

Optimization Tips

1Use --api flag when creating Rails app to minimize middleware and assets.

2Avoid serving HTML views if only JSON APIs are needed to reduce payload size.

3Check network response size and time to verify API-only performance gains.

Performance Quiz - 3 Questions

What is the main performance benefit of using Rails API-only mode?

A Improves client-side rendering by adding more JavaScript

B Automatically caches frontend assets for faster loading

C Smaller server response size by excluding views and assets

D Enables server-side rendering of React components

DevTools: Network

How to check: Open DevTools > Network tab, reload API request, and inspect response size and time

What to look for: Look for smaller response payload and faster time to first byte (TTFB) in API-only mode