Do you mean in the schema or in the view? There is a difference. The size of the schema has less of an impact as does the number of controls in the view. This is even more the case when working with web forms. With web forms, you want to keep the controls in the view to a minimum. But then there's a lot of perf issues with web forms that is way outside the scope of this thread, and have been discussed in multiple threads if you want to take time to search on that.
The amount of data in your XML will have a bigger impact than the size of the schema. So the order of perf impact would be: Controls in view, Size of data, Size of schema. The schema will have the least impact on perf.
Hope that helps some.