How can I memoize a gradient mask in Flutter? - performance

Question

I made an Android app long time ago where the main focus were a bunch of differently colored icon buttons representing psychological emotions. I realized that a few people I had it tested with were slightly confused about the icons, as they unconsciously associated different colors to these emotions than the ones I had chosen.
So, now I'm rewriting the app with Flutter, and I've decided that these icon buttons will have gentle gradients instead of solid colors, as it seems that specific color relationships, rather than colors, are more consistently associated with the emotions, plus it encourages association by learning instead of usage of innate ideas, since it makes the appearance of each icon harder to completely describe but easier to distinguish from the others.
Anyway, I'm using a ShaderMask with a LinearGradient, and an IconButton as child. Obviously it eats up GPU and rendering time is slightly above 16 ms. I thought of memoizing the rendered icon with the gradient, by pre-rendering it to an image then using it in the buttons. I've seen a good increase in performance, but the resulting code felt so hackish and buggy that I just reverted everything.
Is there a way to do this without feeling like I'm fighting the framework? Like, an official way to pre-render widgets, or at least to tell Flutter when something can be re-used without changes (performance is terrible even when updating a widget that doesn't have anything to do with these buttons).
Edit: memoizing these gradient icons would also allow me to use them elsewhere, increasing overall consistency whenever these emotions must be represented. Doing this right now would simply nuke performance.
Edit: code:
class MyButton extends StatelessWidget {
MyButton({this.index, this.constraints});
final int index;
final BoxConstraints constraints;
#override
Widget build(BuildContext context) {
return GradientMask(
child: IconButton(
onPressed: () {},
iconSize: constraints.maxWidth,
padding: EdgeInsets.all(0.0),
color: Colors.white,
icon: iconList[index],
),
index: index,
);
}
}
class GradientMask extends StatelessWidget {
GradientMask({this.index, this.child});
final int index;
final Widget child;
#override
Widget build(BuildContext context) {
return ShaderMask(
shaderCallback: (Rect bounds) => LinearGradient(
begin: Alignment.bottomLeft,
end: Alignment.topRight,
colors: gradientList[index],
tileMode: TileMode.repeated,
).createShader(Rect.fromLTWH(0.0, 0.0, 100.0, 100.0)),
child: child,
);
}
}