Why DTOs matter in Spring Boot - Performance Evidence

Performance: Why DTOs matter

MEDIUM IMPACT

This affects the speed of data transfer between server and client and the efficiency of rendering data in the UI.

Sending data from backend to frontend in a Spring Boot application

Spring Boot

public class UserDTO {
  private String username;
  private String email;
  // getters and setters
}

@GetMapping("/user/{id}")
public UserDTO getUser(@PathVariable Long id) {
  UserEntity user = userRepository.findById(id).orElse(null);
  if (user == null) return null;
  UserDTO dto = new UserDTO();
  dto.setUsername(user.getUsername());
  dto.setEmail(user.getEmail());
  return dto;
}

DTO sends only needed fields, reducing payload size and avoiding sensitive data exposure.

📈 Performance GainSaves 50-150ms on LCP by reducing data transfer and parsing time.

Sending data from backend to frontend in a Spring Boot application

Spring Boot

public class UserEntity {
  private Long id;
  private String username;
  private String password;
  private String email;
  private Date createdAt;
  // getters and setters
}

// Controller returns UserEntity directly
@GetMapping("/user/{id}")
public UserEntity getUser(@PathVariable Long id) {
  return userRepository.findById(id).orElse(null);
}

Sending entire entity exposes sensitive data and sends unnecessary fields, increasing payload size and slowing rendering.

📉 Performance CostAdds extra KBs to response size, increasing LCP by 100-200ms depending on network.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Sending full entity	No extra DOM nodes	No reflows from data	Higher paint cost due to larger data	[X] Bad
Sending DTO with minimal fields	No extra DOM nodes	No reflows from data	Lower paint cost due to smaller data	[OK] Good

Rendering Pipeline

When the backend sends data, the browser parses JSON and renders content. Smaller payloads reduce parsing and rendering time.

→Network Transfer

→Parsing

→Rendering

⚠️ BottleneckNetwork Transfer and Parsing

Core Web Vital Affected

LCP

This affects the speed of data transfer between server and client and the efficiency of rendering data in the UI.

Optimization Tips

1Always send only the data needed by the frontend using DTOs.

2Avoid exposing sensitive or large fields in API responses.

3Smaller payloads reduce network time and improve loading speed.

Performance Quiz - 3 Questions

Why does using DTOs improve page load speed in a Spring Boot app?

A They add more fields to the response

B They reduce the amount of data sent over the network

C They increase the number of database queries

D They delay server response intentionally

DevTools: Network

How to check: Open DevTools, go to Network tab, reload the page, select the API call, and check the response size and timing.

What to look for: Look for smaller response payload size and faster time to first byte indicating better performance.