Flutter code creating laggy animation. What is wrong here?

The whole purpose of using AnimationController is to listen to its events - that's what AnimatedBuilder does and rebuilds its subtree accordingly.

I will post here my overall recommendations on what to change in the code.

Remove setState - that's what makes your entire layout rebuild all over again, i.e. lags.

Also trigger _animeController listeners, i.e. AnimatedBuilder in your case - to rebuild itself.

accelerometerEvents.listen((AccelerometerEvent event) {  var a = ((event.x * 100).round() / 100).clamp(-1.0, 1.0) * -1;  var b = ((event.y * 100).round() / 100).clamp(-1.0, 1.0);  if ((x - a).abs() > 0.02 || (y - b).abs() > 0.02) {    x = a; y = b;    _animeController.value = _animeController.value; // Trigger controller's listeners  }});

Start animation in initState, instead of build. This is the second thing that produces lags in your case. .forward triggers rebuild of your widgets, which leads to an infinite loop.

@overridevoid initState() {  super.initState();  _animeController.forward();}

Use child property of your AnimatedBuilder to save up resources on rebuilding the avatar block every time. Also I'm not sure what Scaffold is here for - let's remove it if it's not needed.

AnimatedBuilder(  animation: _animeController,  builder: (context, child) => Transform(    transform: Matrix4.translationValues(_anime.value * width * x, _anime.value * height * y, 0.0),    child: child,  ),  child: Center(    child: CircleAvatar(      radius: 15.0,      backgroundColor: Colors.green,    ),  ),);

Follow Official Animations Tutorial to master Flutter animations.

Let me know if this helps.