It's hard to tell. What properties are you requestion and which objects? That may dictate sizing and drive long queries. Performance on vCenter or your network can affect it as well.
Have you reviewed the logs on vCenter for any errors?
Can you do a simple RetrievePropertiesEx with just VM names or something simple? Does that hang?
Is it hanging or just taking a long time to return?
You can use WaitForUpdatesEx() and check for multiple updates, that will let you adjust the object set size by modifying some of the parameters.