Any chance you know of someone within Epicor that has some say in Report Data Definitions. Pretty please can you ask them to speed it up. Anything you do in the RDD are just so SLOW and it it feels like I am moving backwards. I was hoping there was a chance it would speed up in 10.2.200 but I am sad to say that is not the case.
Try to exclude columns not being used from the various tables. Make sure your table relationships are based on existing relationships first check the existing links before creating your own.