Raster

The raster thread (previously known as the GPU thread) executes 
graphics code from the Flutter Engine.
This thread takes the layer tree and displays it by talking to
the GPU (graphic processing unit). You cannot directly access
the raster thread or its data, but if this thread is slow, it’s a
result of something you’ve done in the Dart code. Skia, the
graphics library, runs on this thread.

Sometimes a scene results in a layer tree that is easy to construct,
but expensive to render on the raster thread. In this case, you
need to figure out what your code is doing that is causing
rendering code to be slow. Specific kinds of workloads are more
difficult for the GPU. They might involve unnecessary calls to
saveLayer(), intersecting opacities with multiple objects,
and clips or shadows in specific situations.

For more information on profiling, check out
Identifying problems in the GPU graph.

Jank (slow frame)