Each frame should be created and displayed within 1/60th of
a second (approximately 16ms). A frame exceeding this limit
(in either graph) fails to display, resulting in jank,
and a vertical red bar appears in one or both of the graphs.
If a red bar appears in the UI graph, the Dart code is too
expensive. If a red vertical bar appears in the GPU graph,
the scene is too complicated to render quickly.

The vertical red bars indicate that the current frame is
expensive to both render and paint.When both graphs
display red, start by diagnosing the UI thread.

Flutter’s threads

Flutter uses several threads to do its work, though
only two of the threads are shown in the overlay.
All of your Dart code runs on the UI thread.
Although you have no direct access to any other thread,
your actions on the UI thread have performance consequences
on other threads.