These games seem to constantly have unused vertices, and this is worst shown in the Shadow Pokemon purification cutscene. The Shadow Pokemon purification cutscene is even worse on XD with forced single core mode, as instead of having FPS dropping with VPS staying 60ish, it will drop both, resulting in audio stuttering. Turning on CPUCall seems to have a 7/8 reduction of draw calls for that cutscene (~800 -> ~100), doubling performance. Many other areas of the game seem to benefit from this setting too, having some kind of performance boost.
CasualPokePlayer