Earlier quoted context omitted.
The major improvement for everyone is decreased startup time thanks to Multicore JIT. That was one of my few irritations with Paint.NET. Paint.NET should perform better overal thanks to GPU acceleration and multithreaded rendering. I've personally used the alpha's and beta's for a while now and it just feels zippier.
For several years I had put off using Paint.NET over GIMP due to the dependency on .NET, but as soon as I realized that it still managed to outperform GIMP, I moved on the spot. Still do not get how GIMP gets compiled on Windows to perform so poorly.
In this case, GIMP is fully POSIX and GTK based so it needs to translate all POSIX stuff for Windows + rely on a subpar implementation of GTK for the Windows platform.
A good example of how native vs managed usually impacts performance much less than the application structure.