Check out http://john.beynon.org.uk/2009/02/11/jquery-validation-remote-attribute/ - you can tell jQuery to go to a URL to validate a field (almost) automatically. This would make a dramatic difference in your performance, as that 7MB would never have to leave the server. (You can always leverage Spidermonkey to run your current validation JS on the server if need be, but unless it's hideously complex I'd redo it in Perl.)

A quick Google search on 'jquery remote validation' turned this right up.