Hi,
exactly same observations here, so it seems to be the way it is. I've explicitly made sure name resolution is instantaneous on all sides and in both directions (due to having GRE issues that might have been DNS related but weren't - the usual AD access rights shenanigans) and this didn't change the initial deep-thought phase of the backup. It simply seems that enumerating the stuff on the Exchange side of things takes a lot of time, even when you just expand the source tree of the backup spec. It's running and even performing well when it comes up to speed once, so I learned to live with it.
HTH,
Andre.