FYI:
.NET Framework 2.0, 3.0, and 3.5 uses the same versions of the CLR, namely 2.0.
Though, .NET 3.5 installs .NET 2.0 SP1 alongside, providing performance improvements for .NET 2.0-targeted applications, like Paint.NET.
Paint.NET does not make use of 3.0 or 3.5 additional librairies.
So, theorically, you could get the same improvements under a .NET 2.0 with SP1 installation.
You didn't understand anything?
Neither do I.