Swagger API documentation in NestJS - Performance & Optimization

Performance: Swagger API documentation

MEDIUM IMPACT

This affects the initial page load speed and bundle size by adding metadata and UI assets for API docs.

Serving API documentation in a NestJS app

NestJS

import { SwaggerModule, DocumentBuilder } from '@nestjs/swagger';

const config = new DocumentBuilder()
  .setTitle('API')
  .setDescription('API docs')
  .setVersion('1.0')
  .build();
const document = SwaggerModule.createDocument(app, config);
if (process.env.NODE_ENV === 'development') {
  SwaggerModule.setup('api', app, document);
}

Only enables Swagger UI in development, avoiding extra assets and render blocking in production.

📈 Performance GainSaves ~300kb in production bundle, improves LCP by avoiding render-blocking UI

Serving API documentation in a NestJS app

NestJS

import { SwaggerModule, DocumentBuilder } from '@nestjs/swagger';

const config = new DocumentBuilder()
  .setTitle('API')
  .setDescription('API docs')
  .setVersion('1.0')
  .build();
const document = SwaggerModule.createDocument(app, config);
SwaggerModule.setup('api', app, document);

This loads the full Swagger UI and JSON docs on every app start, increasing initial bundle size and blocking rendering.

📉 Performance CostAdds ~300kb to bundle, blocks rendering for 100-200ms on initial load

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Full Swagger UI always loaded	High (many UI nodes)	Multiple reflows on load	High paint cost due to UI	[X] Bad
Swagger UI loaded only in development	Low (no UI in prod)	No reflows in prod	Minimal paint cost in prod	[OK] Good

Rendering Pipeline

Swagger UI assets and JSON docs are loaded after initial HTML, triggering style calculation, layout, paint, and composite steps. Large JSON or UI scripts increase load and block rendering.

→Style Calculation

→Layout

→Paint

→Composite

⚠️ BottleneckPaint and Composite due to large UI scripts and styles

Core Web Vital Affected

LCP

This affects the initial page load speed and bundle size by adding metadata and UI assets for API docs.

Optimization Tips

1Only enable Swagger UI in development or behind authentication to avoid extra load in production.

2Minimize Swagger JSON size to reduce parsing and rendering time.

3Use lazy loading or conditional setup to prevent blocking the main thread.

Performance Quiz - 3 Questions

What is the main performance impact of including Swagger UI in a NestJS app?

A Increases initial bundle size and blocks rendering

B Improves API response time

C Reduces server CPU usage

D Decreases network latency

DevTools: Network

How to check: Open DevTools, go to Network tab, filter by JS and JSON, reload page and observe Swagger UI assets loading

What to look for: Look for large Swagger UI JS and JSON files loading and blocking main thread; absence in production indicates good performance