You are right. Bug is not the correct term here, sorry.
What I tried to say (sorry, my native language is not English) was:
is it possible to improve the selection animation so that it does not use that much CPU?
There is a bug in Paint.NET where if you select something, there is an enormous CPU usage.
I think it has something to do with the dashed line animation (the borders marking your selection) which it then shows.