It is important to note that MS recommends nowadays to make apps in the Windows App SDK. It allows them to produce apps compatible with the MS Store and the MSIX packaging style to install itself "like" a UWP app. But it is not a UWP app.
WASDK apps can be made using either Win32 (GDI32/USER32 API) or .NET (WPF/WinForms API). This is why pre-existing apps can just be added to the Store as they have already been made in Win32 style. Honestly it was a good choice on their part; it helps unify "UWP" (which was more notable as just WPF apps), and Win32 apps. As far as performance, both are pretty much the same. Win32 uses C++ (or anything really, if you use the bindings), and .NET uses C#.
As far as how easy it is, undoubtedly it is easier for a novice programmer to learn .NET than to learn Win32. Win32 is archaic, and while that does not make it worse (in fact, it could be more performant), it makes it less appealing as once again, time = money, and whichever saves the most amount of money is what MS will pick. .NET is easier and quicker to produce apps in.
The performance problem with Windows 11 is a result of the transitional period of moving everything to use WASDK. It is an amalgamation of decades worth of APIs and ways to make an app, and MS took this long to hopefully even decide on an app format they will keep. Windows 7 still remains the last version they produced that was unified, and retained one main format to make apps. Windows 8 added UWP, and Windows 10 smashed them all together into a melting pot. Windows 11 added another one and just fueled the fire that is the melting pot of app formats.
TL;DR: Performance issues on Windows 11 are a result of years of indecisive choices of what app format MS wants developers to use. Windows right now is atrocious, taking the time to learn Linux and alternatives to software you use is so worth it and your sanity.