Example app that shows how having a lot of textboxes can be slow

textbox_slow.dart

// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: SingleChildScrollView(
        child: Column(
          children: <Widget>[
            for (int i = 0; i < 13; i++)
              Row(
                children: [
                  SizedBox(height: 200),
                  Text('$i'),
                  for (int j = 0; j < 10; j++) 
                    Padding(
                      padding: const EdgeInsets.all(4.0),
                      child: SizedBox(width: 50, child:TextField()),
                    )
                ],
              ),
          ],
        ),
      ),
    );
  }
}