Well, it appears I've found the problem. Our app has a fairly decent-sized SET PROC list. If I issue a SET PROCEDURE TO, then the chart will render within a second.

So, the $1,000,000 question is why would the SET PROC list have any negative impact on GDI+?

And the bonus question would be why this issue doesn't even show up when stepping through the debugger while the same SET PROC list is in effect.

Either way, I'm thrilled that I found the issue!