That's the first example of "not fast" that came to my mind actually.

Seriously? The effort Rick puts into Paint.NET performance is unreal (check out his Twitter sometime). I've found it to be impressively fast, especially for how much functionality it has.

FWIW it was only recently migrated to a modern .NET version, give the latest version a spin if you haven't already.

Paint.NET is a good example for how much effort it takes to develop a .NET app that has decent performance especially once it is a little bit complex

Just because it's impressive doesn't mean it's not slower than other systems. It's not mutually exclusive.

Agreed. Rick’s work on Paint.NET is just so impressive.

Odd, I've been using it for the better part of a decade when GIMP is too much and I've never felt like it was slow.

Indeed, if anything is slow, it's GIMP.