I'm getting the same problem, on high quality leader detail it freezes/crashes, on any other detail it takes a minute or so to open the leader screen and nearly always there's no animation. And then a further minute or so opening each subsection of the diplomacy screen for things like trade, pacts, etc.
Lowering the leader detail quality to minimum pretty much does nothing to alter performance, hardly any change at all.
Using i920 OC'd to 3.4ghz, GTX295 in SLI, 12GBddr3 RAM, Vista 64bit. I'd certainly not expect problems at minimum detail on this machine.
It's strange, because the rest of the game runs flawlessly with no turn delay with all graphical options set to max, so there definitely seems to be some issue with the leader graphics.
Does anyone know what (if any) other graphical options effect this, and maybe I can try turning those down.
Just about to check for updates to graphic drivers, don't really like updating them unless I run into problems as nvidia quality's been a bit hit and miss recently, so when I find on that works I tend to stick with it.
Edit: Actually I'll check for driver updates tomorrow, just realised it's nearly 3.30 am here!