No - all properties are just tied wrappers for Javascript objects living on the other side in Firefox. Every access and function call has to travel from Perl to Firefox and back, and that is slow.

If you are interested in the nasty details, I think that MozRepl::RemoteObject somewhat goes into the nasty details, and I gave one talk on the subject, albeit in German only. I'm not sure if the Google translation of the slides are usable...