[dagw]

Paint.NET is fine for small images, but performance really falls off a cliff when images get larger, whereas Photoshop hardly misses a beat. Which I guess is kind of your point, for doing 'normal' tasks under 'normal' loads C# is fine, but if you have to go beyond normal, then you really need the control and speed of something C-like.

[nxn]

I don't know, to me it seems there's a likely chance that Photoshop's implementation of handling large files is just better, and if Paint.NET took a similar approach maybe the performance might be more comparable. By this I mean stuff like not having to go through the whole file to generate a zoomed view of it, etc. Of course this is just an assumption based on the fact Paint.NET isn't in the same class of application as Photoshop, so I wouldn't expect handling massive files to be a priority to the authors. That and Photoshop has been around a lot longer and Adobe probably has more resources to throw at potential problems like this. Either way, these are just guesses to consider before assuming the difference is caused by the language.