Because a GWT is based on AJAX, it is likely to perform slightly better than either
Struts or
JSF, mainly because it makes individual updates to the page, rather than refreshing the entire page.
However, because much more code runs on the browser in a GWT application, how well it performs will depend on the speed of the client system (i.e. the PC running the browser).
Regarding startup time, I would argue that it's not something
you should worry about. In most production systems, the server starts up once and is rarely ever stopped except to do maintenance. What does it matter if it takes the web application a while to load up on the server? It's only going to happen once.