The raster 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.
    Shown in the top row of the performance overlay.
    This thread was previously known as the “GPU thread” because it
    rasterizes for the GPU. But it is running on the CPU. We renamed it
    to “raster thread” because many developers wrongly (but understandably)
    assumed the thread runs on the GPU unit.

Performs expensive tasks (mostly I/O) that would
    otherwise block either the UI or raster threads.
    This thread is not shown in the performance overlay.

For links to more information and videos,
see The Framework architecture on the
GitHub wiki, and the community article,
The Layer Cake.

Displaying the performance overlay