For the normal case (pass-by-value semantics), it should be about the same
speed as Perl 5, since it would be doing the same thing: making a copy of a
value that was passed by reference.

For the "is ro" or "is ref" case, it should be FASTER, since it could avoid
the copy by aliasing the reference.

Only the "is rw" case would be slower, since it would involve two copies
rather than one.  However, it would offer a benefit of "rolling back" any
changes to the parameters in case of an abnormal exit from the function.  
This seems like a valuable feature that could often be useful -- and anyone
who wants to avoid the speed penalty could stick with "is ref", assuming
that option is also allowed...

Deven