CupertinoPageRoute includes a partially-transparent ModalBarrier behind it, as well as a drop-shadow on its left edge. This is to match the native page transition while a new page slides over on top of the current one. But this occurs even on the bottom/first route in the Navigator, where both the shadow and the background fill should never be visible to the user. So this increases raster time and/or memory usage.

CupertinoModalBarrier

(rectangle of 0x00000018)

CupertinoPageTransition

Example code

import 'package:flutter/cupertino.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return const Padding(
      padding: EdgeInsets.all(64),
      child: CupertinoApp(
        home: CupertinoPageScaffold(
          child: Center(
            child: Text('content')
          )
        )
      )
    );
  }
}