MVC is just one architectural pattern you could use. A critique of MVC is that reusability is difficult to achieve. Another critique of MVC is the paradigm locks engineers into a way of working that isn't applicable for every situation when developing applications. For UI development, MVC provides a good separation of concerns for the task it's supposed to achieve: binding a data model to the view.
With or without MVC, but speaking in the parlance of MVC, performance is probably impacted by the functions that bind the "model" to the "view" or the methods that transform the model. How any paradigm implements binding a data model to the view is of great importance for performance. How complex types are iterated upon could contribute to performance issues in any paradigm. Security is also of concern, as there could be potential exploits lurking in some implementation.
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