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The document clearly defines the purpose of
Provides the base class for defining a new control that encapsulates related existing controls and provides its own logic.
I understand your needs. If a XAML file contains too much code, it will be a challenge for subsequent maintenance and updates. It is a good idea to split the page and divide the page into several modules. And of course, you could use
UserControl to do it. You could also try other ways like using Resource Dictionary for styles and templates.
For the second question, there is no official document mentions about the performance loss when using
UserControl. There should be no problem using it.