JSF is certainly slower than
Struts -- but I do not think the overhead JSF adds is that important in the grand scheme of things. With a dual core machine, I can easily drive a JSF+Seam+EJB3+DB application to more than 200 transactions per second -- that is much faster than most developers ever need. In the real world, the performance bottleneck is probably from other layers of the app.
Re Grails: I think you do pay a price in performance for using those cute "dynamic languages"
BTW, Seam 1.3 will provide Groovy integration as well. So, you can much of the same things as Grails in Seam!