This is most likely the "infamous" SP1 vs. original issue...there are issues in the WIM's such that for an SP1 version of windows 7, you need to use the windows 7 version of the tools, not the vista version. This is why there is all the "fuss" about using extracted files...The older version doesn't work right with the WIMs of the SP1 releases. Yes, the older versions of the tools "mount" much faster! the new versions create all sorts of actual disk "links" (for lack of a more proper term) into the WIM, and take a long time to mount the files.
I use the one from the SP1 update of the windows 7 WAIK (6.1.7600.16385) I think there are some other posts here in the forum - I know this was discussed when SP1 first came out...
In general, best to use newest version of the tools, since there are often things like this where the new tool works on older data, but not the other way around.
Glad we (hopefully) got to the bottom of this for you