Flutterexperts

In the world of mobile app development, performance is paramount. A slow-loading application can lead to high uninstallation rates and poor user reviews. For large Flutter applications, especially those targeting Android and Web platforms, techniques like code splitting and lazy loading are essential for reducing initial bundle size and significantly improving app startup times.

This blog post will dive deep into how Flutter handles these concepts, introduction, what code splitting is, why startup becomes slow, Flutter’s lazy loading patterns, route-based loading, deferred imports, and real, production-grade examples, and walk you through the implementation steps for an optimized user experience.

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Table Of Contents:

Introduction

Why Flutter Apps Become Slow at Startup

What is Code Splitting in Flutter?

Lazy Loading vs Deferred Loading

Example Scenario: A Growing App

Code Splitting via Route-Based Lazy Loading

Lazy Loading Heavy UI Components

Benefits of Deferred Loading

Deferred Loading — Flutter’s Most Powerful Code-Splitting Tool

Splitting a Heavy Feature (Chat Module)

Lazy Loading Controllers, Loading Images & Assets

Performance Gains (Realistic Numbers)

Final Example: Clean Architecture + Lazy Loading Setup

Conclusion

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Introduction

Mobile users expect apps to open instantly. Every millisecond matters—especially on mid-range Android devices where startup time directly impacts user retention. As your Flutter project grows, so does the app size, widget tree complexity, initialization logic, and asset loading overhead.

That’s where Code Splitting and Lazy Loading come in.

These techniques break your app into smaller, self-contained chunks so that only the minimum code needed for the first screen loads at startup. Everything else loads on-demand, improving:

• Time To First Render (TTFR)
• Perceived performance
• Smoothness of navigation
• Memory efficiency

Why Flutter Apps Become Slow at Startup

Even though Flutter compiles to native ARM code, a large project still suffers from these bottlenecks:

Large Widget Tree: If your app builds multiple heavy screens during startup (e.g., dashboard, analytics, maps), the initial layout and paint operations slow down the time it takes the user to see the UI.

Too Many Global Providers / Controllers:-

Developers often initialize:

• 10+ Riverpod providers
• Firebase
• Local DB (Hive, Drift)
• Analytics SDKs
• Task managers

All this happens before runApp() finishes building the UI.

Large Assets & Fonts:-

Huge images, custom fonts, and lottie animations increase load time because Flutter bundles and parses them before usable UI is rendered.

Codebase Monolith:- When everything is bundled as a single chunk:

• The binary becomes large.
• Isolates cannot load minimal code.
• Memory footprint increases.

Solution → Code splitting + Lazy loading.

What is Code Splitting in Flutter?

Code splitting means dividing your codebase into small chunks instead of one monolithic binary. Flutter can load these chunks only when needed, not at startup.

Flutter supports two major techniques:

1. Route-based code splitting using separate Dart files

(Load screens only when navigated.)

2. Lazy loading using FutureBuilder or async controllers

3. Deferred Imports

Flutter’s most powerful lazy-load mechanism.

Lazy Loading vs Deferred Loading

Feature	Lazy Loading	Deferred Loading
When used	Load UI/data/controllers when needed	Load Dart CODE only when needed
Helps with	Runtime performance	App size & startup time
Example	Fetching list only after button click	Loading a module like “Chat” only when user opens chat

Both work beautifully together.

Example Scenario: A Growing App

Your app has:

• Dashboard (default)
• Chat module
• Analytics module
• Settings
• AI assistant

Most users open dashboard first. So Chat, Analytics, and AI screens do NOT need to load during startup.

Let’s split them into separate modules and load when required.

Code Splitting via Route-Based Lazy Loading

Instead of importing all screens in main.dart:

import 'dashboard.dart';
import 'chat.dart';
import 'analytics.dart';
import 'settings.dart';
import 'ai_assistant.dart';

We only load the entry screen:

import 'dashboard.dart';

Other pages will be loaded lazily using named routes.

main.dart

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Lazy App',
      home: DashboardScreen(),
      onGenerateRoute: (settings) {
        switch (settings.name) {
          case '/chat':
            return MaterialPageRoute(
              builder: (_) => ChatScreen(),   // loads file only when used
            );
          case '/analytics':
            return MaterialPageRoute(
              builder: (_) => AnalyticsScreen();
            );
          default:
            return MaterialPageRoute(builder: (_) => DashboardScreen());
        }
      },
    );
  }
}

How does this help?

• Flutter loads chat.dart only when /chat route is used.
• The startup bundle only contains dashboard code.

Lazy Loading Heavy UI Components

You can avoid building heavy UI until needed.

Example: load analytics chart only after user taps a button.

class AnalyticsScreen extends StatefulWidget {
  @override
  _AnalyticsScreenState createState() => _AnalyticsScreenState();
}

class _AnalyticsScreenState extends State<AnalyticsScreen> {
  bool loadChart = false;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Analytics")),
      body: Center(
        child: loadChart
            ? FutureBuilder(
                future: loadAnalyticsData(),
                builder: (context, snapshot) {
                  if (!snapshot.hasData) return CircularProgressIndicator();
                  return ChartWidget(data: snapshot.data!);
                },
              )
            : ElevatedButton(
                onPressed: () => setState(() => loadChart = true),
                child: Text("Load chart"),
              ),
      ),
    );
  }
}

Benefits of Deferred Loading

• Reduced Initial Download Size: Users only download the core app needed for the first launch, reducing the barrier to entry.
• Faster Startup Times: Less code to initialize at launch means a snappier app start.
• Optimized Resource Usage: Unused features or large assets aren’t loaded into memory unless explicitly requested.
• On-Demand Features: Allows for optional or premium features to be downloaded only by users who need them (e.g., game levels, specific language packs).

Deferred Loading — Flutter’s Most Powerful Code-Splitting Tool

Deferred loading allows Dart to load libraries only when requested, reducing initial binary load.

Use Case:– You want to load the entire Chat module only when the user goes to Chat.

Example Folder Structure

lib/
  main.dart
  dashboard/
  chat/   <-- heavy logic here (AI, socket, Firestore, media)

Deferred Import Syntax

import 'package:myapp/chat/chat_screen.dart' deferred as chatModule;

Loading Module on Demand

ElevatedButton(
  onPressed: () async {
    await chatModule.loadLibrary(); // Loads chat module at runtime
    Navigator.push(
      context,
      MaterialPageRoute(builder: (_) => chatModule.ChatScreen()),
    );
  },
  child: Text("Go to Chat"),
)

How it works internally

• Flutter compiles chat code into a separate chunk.
• It is NOT loaded into memory until loadLibrary() is called.
• Reduces startup code size by up to 30–50%, depending on module size.

Splitting a Heavy Feature (Chat Module)

Before (Monolithic Import)

import 'chat_screen.dart'; // loads everything at startup

After (Deferred Loading)

main.dart

import 'package:myapp/chat/chat_screen.dart' deferred as chat;

Button to load/chat

ElevatedButton(
  child: Text("Open Chat"),
  onPressed: () async {
    await chat.loadLibrary();  
    Navigator.push(
      context,
      MaterialPageRoute(builder: (_) => chat.ChatScreen()),
    );
  },
)

Advantages

• Faster cold start
• Lower memory consumption
• Large modules (AI, Chat, Maps, PDF viewer) stay unloaded until needed
• Smaller RAM footprint on low-end devices

Lazy Loading Controllers, Loading Images & Assets

Instead of initializing controllers globally:

❌ Bad

final ChatController chatController = ChatController();  // created at app launch

✔ Good: Initialize when screen opens

class ChatScreen extends StatefulWidget {
  @override
  _ChatScreenState createState() => _ChatScreenState();
}

class _ChatScreenState extends State<ChatScreen> {
  late ChatController controller;

  @override
  void initState() {
    super.initState();
    controller = ChatController();  // created when screen loads
  }
}

Large images delay startup due to asset decode time.

Use:

• FadeInImage.memoryNetwork
• Image.asset("...", cacheWidth: ...)
• precacheImage() only when needed

Lazy loading example:

ListView.builder(
  itemCount: items.length,
  itemBuilder: (_, i) {
    return Image.network(
      items[i].url,
      loadingBuilder: (_, child, progress) =>
          progress == null ? child : CircularProgressIndicator(),
    );
  },
);

Performance Gains (Realistic Numbers)

Feature	Before Lazy Loading	After Lazy Loading
Startup time	1.8 sec	0.9 sec
App binary in RAM	200 MB	120 MB
Initial Dart code load	100%	45%
Perceived performance	⭐⭐	⭐⭐⭐⭐⭐

Apps with Maps, Charts, AI, Lottie animations see massive gains.

Final Example: Clean Architecture + Lazy Loading Setup

main.dart

import 'package:flutter/material.dart';
import 'features/dashboard/dashboard.dart';
import 'features/chat/chat.dart' deferred as chat;

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: DashboardScreen(),
    );
  }
}

Dashboard

ElevatedButton(
  onPressed: () async {
    await chat.loadLibrary();
    Navigator.push(
      context,
      MaterialPageRoute(
        builder: (context) => chat.ChatScreen(),
      ),
    );
  },
  child: Text("Open Chat (Lazy Load)"),
)

Chat module stays fully unloaded until user opens Chat.

Conclusion:

In the article, I have explained how Code Splitting & Lazy Loading in Flutter: Speeding Up App Startup Times. This was a small introduction to User Interaction from my side, and it’s working using Flutter.

If your Flutter app has grown over time, chances are your startup time has slowed down. By applying code splitting, lazy loading, and deferred imports, you can:

• Reduce initial load time
• Lower memory usage
• Load only what is necessary
• Improve performance on low-end devices
• Make your app feel instant and modern

These strategies are no longer optional—they are essential for production-ready Flutter apps in 2025 and beyond.

❤ ❤ Thanks for reading this article ❤❤

If I need to correct something? Let me know in the comments. I would love to improve.

Clap 👏 If this article helps you.

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From Our Parent Company Aeologic

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