There aren't any performance considerations that I can think of off hand that are specific to the usage of the Web 2.0 and Mobile Feature Pack. As with all client/server architectures, you need to be aware of the characteristics of the communications medium that you're using so that you architect your applications in the most effective manner.
Some will claim that HTML/JavaScript/CSS applications will never be as performant as native applications, but we believe, as we did with Java, that any existing issues will be ironed out as the device vendors continue to optimize their runtimes and that the benefits of being able to reuse existing Web development skill sets and share implementations across devices will outweigh the minimal overhead.