Vaadin 6 slow for some, security blocking?

We are currently using Vaadin 6 on our JBoss server, and have been for more 6 months now, for our customer portal.
The users of 1 of our customers complain that it is very, very slow (up to minutes for showing a screen that takes seconds for other customers).
When we log in with their accounts we cannot reproduce the problem.
When they used our old portal, they did not complain about speed issues. Therefor we don’t believe it is due to the network connection.
Therefor we believe it is something with their security setup. The strange thing then is that if they wait long enough, it will eventually work.
Has anybody else came accross such an issue, and what is a possible solution?
Would upgrading to Vaadin 7 solve anything?

Have you checked with the customer which browser they are using and tried to reproduce the issue with the exact same browser? The difference in loading times are staggering between IE6/8 and more modern browsers that are usually used in development.

Thank you for your reply.
Both we and the customer have tried with various browsers (old and new browsers).
That customer reported the issue the first time when they used Google Chrome (I’m not sure which version).

A very old post, but maybe this is still relevant to somebody. If the slowness was on the first load of the application, that would strongly suggest that it is a network issue related to loading the widgetset JavaScript files or the bootstrap page. It could be e.g. some proxy or content inspection server that causes delays there.