I haven't run any tests on this, but my suspicion is that there is very little if any difference in performance between the two types of Actionform bean.
Personally, I don't like DynaForm beans. If you have a good
IDE, it's just as easy to write an ActionForm bean as it is to provide the XML for a DynaForm bean. Once you have an ActionForm bean, you have much more flexibility. For example, you can change a getter to do something other than just retrieve the property. Properties of ActionForm beans are also much easier and more intuitive to retrieve than those of DynaForm beans. Example:
as opposed to
In the first example, the compiler will tell me if I spell userName wrong. It won't tell me if I spell it wrong in the second example.
About the only place I use the DynaForm is when I'm creating a quick-and-dirty prototype application.