Repository: flutter
Issue: SliverAppBar with Google Map lags on expand
Labels: c: performance,f: scrolling,p: maps,package,perf: speed,team-ecosystem,has reproducible steps,P2,found in release: 2.2,triaged-ecosystem

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SliverAppBar with a Google Map is lagging when I expand. Please see a video. Also confirmed on actual device
![ezgif com-optimize](https://user-images.githubusercontent.com/283347/97435142-532c6b80-1920-11eb-9bba-fbbce95be504.gif)

Here is the code

```
import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:google_maps_flutter/google_maps_flutter.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  MyApp({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final title = 'Floating App Bar';

    return MaterialApp(
      title: title,
      home: Scaffold(
        // No appbar provided to the Scaffold, only a body with a
        // CustomScrollView.
        body: CustomScrollView(
          slivers: <Widget>[
            // Add the app bar to the CustomScrollView.
            SliverAppBar(
              // Provide a standard title.
              title: Text(title),
              // Allows the user to reveal the app bar if they begin scrolling
              // back up the list of items.
              floating: true,
              // Display a placeholder widget to visualize the shrinking size.
              flexibleSpace: GoogleMap(
                //polylines: _polyline,
                // YOUR MARKS IN MAP
                initialCameraPosition: CameraPosition(
                    //target: userPosition,
                    //zoom: 15.0,
                    target: LatLng(47.502941, 18.999161),
                    zoom: 1),
                myLocationEnabled: true,
                zoomControlsEnabled: true,
              ),

              // Make the initial height of the SliverAppBar larger than normal.
              expandedHeight: 200,
            ),
            // Next, create a SliverList
            SliverList(
              // Use a delegate to build items as they're scrolled on screen.
              delegate: SliverChildBuilderDelegate(
                // The builder function returns a ListTile with a title that
                // displays the index of the current item.
                (context, index) => ListTile(title: Text('Item #$index')),
                // Builds 1000 ListTiles
                childCount: 1000,
              ),
            ),
          ],
        ),
      ),
    );
  }
}


```

Output of flutter doctor
```

Microsoft Windows [Version 10.0.16299.2166]
(c) 2017 Microsoft Corporation. All rights reserved.
C:\Users\.... \AndroidStudioProjects\....>flutter doctor -v
[√] Flutter (Channel beta, 1.23.0-18.1.pre, on Microsoft Windows [Version 10.0.16299.2166], locale hu-HU)
    • Flutter version 1.23.0-18.1.pre at C:\flutter
    • Framework revision 198df796aa (13 days ago), 2020-10-15 12:04:33 -0700
    • Engine revision 1d12d82d9c
    • Dart version 2.11.0 (build 2.11.0-213.1.beta)


[√] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
    • Android SDK at C:\Users\....\AppData\Local\Android\Sdk
    • Platform android-30, build-tools 30.0.2
    • Java binary at: C:\Program Files\Android\Android Studio1\jre\bin\java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)
    • All Android licenses accepted.

[√] Chrome - develop for the web
    • Chrome at C:\Program Files (x86)\Google\Chrome\Application\chrome.exe

[√] Android Studio (version 4.1.0)
    • Android Studio at C:\Program Files\Android\Android Studio1
    • Flutter plugin can be installed from:
       https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
       https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)

[√] IntelliJ IDEA Community Edition (version 2020.2)
    • IntelliJ at C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2020.2.2
    • Flutter plugin installed
    • Dart plugin version 202.7319.5

[√] VS Code (version 1.50.1)
    • VS Code at C:\Users\.....\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.15.1

[√] Connected device (3 available)
    • Android SDK built for x86 (mobile) • emulator-5554 • android-x86    • Android 10 (API 29) (emulator)
    • Web Server (web)                   • web-server    • web-javascript • Flutter Tools
    • Chrome (web)                       • chrome        • web-javascript • unknown

• No issues found!


```