Flutter app becomes much slower by just one widget - dart

Question

So in my app I have a simple rating feature and in order to achieve this I've built a simple star widget which takes a 'fill' parameter 0.0-1.0 and the Rating widget uses a row of five stars counting the appropriate fill. The problem is that for some unexplainable reason the list (cuz it appears on a list) becomes so much slower when that specific widget is removed. Here's the code for he star:
class Star extends StatelessWidget {
final double fill;
final double scale;
Star({#required this.fill, this.scale = 1.0}):
assert(fill<=1.0 && fill >=0.0);
#override
Widget build(BuildContext context) {
return new Container(
margin: new EdgeInsets.only(
right: 2.5*scale,
),
child: new Material(
elevation: 2.0,
borderRadius: new BorderRadius.circular(3.0),
child: new Container(
height: 14.0*scale,
width: 14.0*scale,
child: new Stack(
children: <Widget>[
new Container(
color: Colors.blue,
width: this.fill * 14.0*scale,
),
new Center(
child: new Material(
elevation: 3.0,
color: new Color(0x00000000),
child: new Icon(
Icons.star,
size: 12.0*scale,
color: Colors.white,
),
),
),
],
),
),
),
);
}
}
And for the full rating widget:
class Rating extends StatelessWidget {
final double rating;
final double scale;
Rating({
#required this.rating,
this.scale = 1.0,
})
: assert(rating <= 5 && rating >= 0);
#override
Widget build(BuildContext context) {
final List<Star> stars = new List.generate(5, (index) {
return new Star(
fill: _getFill(index),
scale: scale,
);
});
return new Row(
children: stars,
);
}
int _getStarsNeeded() {
double r = rating;
if (r <= 0)
return 0;
else if (r > 0 && r <= 1)
return 1;
else if (r > 1 && r <= 2)
return 2;
else if (r > 2 && r <= 3)
return 3;
else if (r > 3 && r <= 4)
return 4;
else
return 5;
}
double _getEndingStarFill() {
return (rating + 1 - _getStarsNeeded());
}
double _getFill(index) {
int neededStars = _getStarsNeeded();
if (index < neededStars - 1) {
return 1.0;
} else if (index == neededStars - 1) {
return _getEndingStarFill();
} else
return 0.0;
}
}
I know this is not the best way to find the fill but that can't be the case here. And yet looks so simple to make things heavy.