hmm don't know GWT and your form code doesn't seem to be related to the following stuff, but I have seen the mentioned 5 seconds in a similar context.
at least with the URLFetch service there is a default timeout of 5 seconds and there you have to do something like:
// default is 5 seconds, currently capped at 60 seconds
fetch = URLFetchServiceFactory.getURLFetchService().fetch(
new HTTPRequest(new URL(url), HTTPMethod.GET,
com.google.appengine.api.urlfetch.FetchOptions.Builder
.withDeadline(300)));