A possible reason is that the pz3 contains references to files with a different path (example would be the V4 files which were stored in the P7 runtime). If Poser cannot find it in the path which is references is goes on a hunt to locate tham in all the runtimes which are attached. If those runtimes are very large, it might take a very long time.

One thing you can do (I think this was introduced in P7) is to limit the search range by setting the search to "shallow" in the General Prefences. It will then only look in the most logical places in all the runtimes and give you an error when it is not there. At least then you know which files cannot be found. - that is, if that is really the problem