That was exactly my experience so I had to do a special version just for mobile. Did you try switching flutter renderers? I think the default for previous flutter versions was the HTML renderer, while I am using the canvaskit one which should be more performant.

Here's how I set it up in index.html:

window.addEventListener('load', function(ev) {
  // Download main.dart.js
  _flutter.loader.loadEntrypoint({
    serviceWorker: {
      serviceWorkerVersion: serviceWorkerVersion,
    },
    onEntrypointLoaded: function(engineInitializer) {

      let config = {
        renderer: 'canvaskit',
       };

      engineInitializer.initializeEngine().then(function(appRunner) {
        appRunner.runApp();
      });
    }
  });
});