Which doesn't answer to my question. You said those plug-ins cause issues with customizr, which, if true, is an important thing. By the way I cannot see a valid reason for that, but it's just my opinion, and you can trash it. :)
There's this simple plug-in: http://wordpress.org/plugins/contact-form-7/
which, for what I know, doesn't cause issues, or at least, never caused slow-down issues to me.
p.s.
it's not visual and your clients still need some html skills to build a form, so propably it's not good for your case.