I have a i5-3427U CPU @ 1.80 Ghz and an Intel HD 4000 internal graphics card.
The selection indicator is the blinking line around a selected area in an image. E.g. it is drawn after the selection tool was used.
The implementation of the selection indicator is pretty poorly implemented regarding performance. Just follow those steps to reproduce:
Monitor your taskmanager while doing the following steps
Get any image data into your clipboad, e.g. make a screenshot
Do Ctrl-N, Ctrl-V many times in Paint.net
You will see, the CPU usage increases