Launch DevTools

DevTools provides features like profiling, examining the heap,
displaying code coverage, enabling the performance overlay,
and a step-by-step debugger.
DevTools’ Timeline view allows you to investigate the
UI performance of your application on a frame-by-frame basis.

Once your app is running in profile mode,
launch DevTools.

The performance overlay

The performance overlay displays statistics in two graphs
that show where time is being spent in your app. If the UI
is janky (skipping frames), these graphs help you figure out why.
The graphs display on top of your running app, but they aren’t
drawn like a normal widget—the Flutter engine itself
paints the overlay and only minimally impacts performance.
Each graph represents the last 300 frames for that thread.

This section describes how to enable the performance overlay
and use it to diagnose the cause of jank in your application.
The following screenshot shows the performance overlay running
on the Flutter Gallery example: