Is the slowness on the first frame, or on the whole animation?
If it’s the whole animation, is clipping causing the slow down?
Maybe there’s an alternative way of drawing the scene that doesn’t
use clipping. For example, overlay opaque corners onto a square
instead of clipping to a rounded rectangle.
If it’s a static scene that’s being faded, rotated, or otherwise
manipulated, a RepaintBoundary might help.

Checking for offscreen layers

The saveLayer method is one of the most expensive methods in
the Flutter framework. It’s useful when applying post-processing
to the scene, but it can slow your app and should be avoided if
you don’t need it.  Even if you don’t call saveLayer explicitly,
implicit calls might happen on your behalf. You can check whether
your scene is using saveLayer with the
PerformanceOverlayLayer.checkerboardOffscreenLayers switch.