on_submit only after ajax-request

If I use the additional setting “on_submit” to execute JavaScript after a form submit, this happens only after the ajax request and not after the submit of the form.
Is there any possibility to execute JavaScript right after the form is submitted? If I have a slow internet connection and/or large file attachments, the ajax request needs some time…

https://wordpress.org/plugins/contact-form-7/