We have our own custom scheduling controls in PracticeStudio. I haven't tried to use Infragistics' controls. You're right, though sounds like they're trying to do some extra ciphering when you group them.
Have you tried to set the CurrentDataTable as the binding source on the control rather than the business object itself. They might be doing some recursive enumerating or something that is more efficient in a DataTable than a BO. That's the only suggestion that I have.