This is a pretty serious bug.
I installed Paint.Net a year or so ago (before 2.5) tried it for a few days, found it too slow, and uninstalled.
Yesterday I decided I'd try again and use it instead of PSP for some screenshots I have to do for work. I Prnt-Scrn-pasted a couple of pictures in (PaintShop Pros Capture function is sorely missed here, but thats a feature req issue).
Then I saw my whole machine slow down. Task manager shows I'm runing a 100%! Not 20%, 100%.
I minimized Paint.Net an saw it went back down to idle.
Today I expanded the window, back up to 100%. I really came close to uninstalling and going back to PSP but decided to check the forum first. Saw this post, did an unselect, back to idle. Amazing.
Take this one seriously. Something as simple as selection eating all my CPU is a deal breaker.
I have a Del D600 1.6GHz with 2 Gig of RAM and a reasonable ATI graphics card (laptop). But frankly, my hardware is irrelevant. If a selection marquee eats 20% of CPU on any machine, it's a bug.
Nice app otherwise. Get me some screen capture and a color match (a la PS) and Im sold