I suspect the difference between a request formatted as a normal GET with a url giving a file name and a GET to a REST server would not be measurable.
A RESTful server such as you can create with the Jersey project does have a little extra interpretation step to steer the request to the right method but it is probably too small to measure. The initial startup of a Jersey server involves extra steps as it locates classes with the correct annotations, but that is just startup time.
In contrast with the overhead of -say- a
SOAP server, which involves parsing XML, REST requests are very simple.
Bill