Which is the most performant way of overriding views field output
I would go for Template based solution for the sake of maintainability and flexibility. For example, if you are using the same field in two different views, you can use a .tpl file that applies to all views that have that field. But if you depend on Views UI, you will have to configure each field in every view separately which may complicate things in the long run. Also, the fewer access to database to retrieve configuration the better.