This can sound stupid maybe, but what about to design for not having the problem in the first stage? (i.e. no corners nor shadows). Also make sure it's a matter of transfer time and not a rendering speed issue.

Btw some day ago I released this little tool called wbox ( http://hping.org/wbox ), probably I good feature to add is to try to fetch all the js/css/images data like a real browser and time it in order to tell what is really taking time.