Do you happen to have the "Include sizes, copyright information, modification dates, etc" option checked on under Inventory Collection Preferences? That will increase collection time so it might be worth checking if that's enabled and if so, try it with the option off.
It sounds like its something else that's going on though. Any Adobe applications on these Macs? 8.7 shouldn't have the issue but older versions often had a problem with certain Adobe apps, such as Adobe Speedgrade, where it could get hung up almost indefinitely on a symlink inside the app bundle.
Also, does it ever finish recon but just takes forever, or are you seeing not complete at all?