Weird performance issue in drawing path using flutter - flutter

Question

I'm trying to draw some pointing triangles on CustomPaint and I'm facing performance issues when I change the shape of the triangle. If you execute the current code the refresh rate is fast.
If you change the part Offset(x,y+8), with Offset(x,y+4), the triangle shape changes slightly but the performance drops and the refresh rate is poor.
Does anyone knows why that happens and how can I resolve this issue?
import 'dart:math' as Math;
import 'package:flutter/material.dart';
void main() =>
runApp(
MaterialApp(
title: 'Demo',
home: Scaffold(
body: Shapes()
),
)
);
class Shapes extends StatefulWidget {
#override
_ShapesState createState() => new _ShapesState();
}
class _ShapesState extends State<Shapes> with SingleTickerProviderStateMixin{
Animation<double> animation;
AnimationController animationController;
#override
void initState() {
super.initState();
animationController = AnimationController(
vsync: this,
duration: Duration(seconds: 5),
)..addListener(() => setState(() {}));
animation = Tween<double>(
begin: 50.0,
end: 120.0,
).animate(animationController);
animationController.forward();
animation.addStatusListener((status) {
if (status == AnimationStatus.completed) {
animationController.repeat();
}
});
}
#override
Widget build(BuildContext context) {
return CustomPaint(painter: ShapesPainter());
}
}
class ShapesPainter extends CustomPainter {
#override
void paint(Canvas canvas, Size size) {
final paint = Paint();
paint.strokeWidth = 2;
for (var i = 0; i < 1000; i++) {
Math.Random rnd = new Math.Random();
double x = 5 + rnd.nextInt(300 - 5).toDouble();
double y = 5 + rnd.nextInt(1200 - 5).toDouble();
final Path path = Path();
path.addPolygon([
Offset(x+6,y+8),
Offset(x,y+-8),
Offset(x-6,y+8),
Offset(x,y+8),
], true);
paint.color = Color(0xFF3f6cbf);
paint.style = PaintingStyle.stroke;
canvas.drawPath(path, paint);
paint.color = Color.fromARGB(255, 255, 0, 0);
paint.style = PaintingStyle.fill;
canvas.drawPath(path, paint);
}
}
#override
bool shouldRepaint(CustomPainter oldDelegate) => true;
}