Issue
When network performance is slow and a postback is needed (such as when the user clicks on a radio button that displays another field), the Submit button still appears during the page load.
Solution
A simple solution is to use client-side JavaScript to hide the submit button in all your web controls that triggers a Page.PostBack.
For example, If you have a RadioButtonList control that upon selection of a value will trigger a PostBack to show or hide the Subject field. In that RadioButtonList's Settings / Custom attributes / Add a new attribute, add onchange with the code to hide the Submit button.