To see the stats about which widgets are being rebuilt, and how often,
click Show widget rebuild information in the Performance pane.
The exact count of the rebuilds for this frame displays in the second
column from the right. For a high number of rebuilds, a yellow spinning
circle displays. The column to the far right shows how many times a
widget was rebuilt since entering the current screen.
For widgets that aren’t rebuilt, a solid grey circle displays.
Otherwise, a grey spinning circle displays.

The app shown in this screenshot has been designed to deliver
  poor performance, and the rebuild profiler gives you a clue
  about what is happening in the frame that might cause poor
  performance. The widget rebuild profiler is not a diagnostic
  tool, by itself, about poor performance.