a simple client-side approach would be to invoke a javascript function when
the form is submitted that unhides a wait message, or to take the idea
further, hides/unhides whole sections of html to make it look like you've
loaded a totally new page. The  response, when it occurs  will load your
final page.

if you want a wait page that displays a percentage done message or graphic,
there is much more work involved. I can provide details of a way to do it if
you want.