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.
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.