FFXI cause the card to kick out from 2D speeds because it calls functions related to T&L (transform and lighting).
What the graphics card is doing mostly is animate particles, do lighting effects on the textured surfaces and perform that "aura" effect which is what cause telepoints and confluxes overheat the GPU.
The aura effect is achieved repeating the same 3D pattern several times in a set distance of pixels...
If you look at the crag telepoints, there's a pink round aura surrounding the telepoint crystal, at the ground...
That's what make the card heat up there. There's severe lighting effects on the edges of the veridical confluxes (golden spots) on both top and lower edges. Along with a soft aura effect that particles effect is what cause it to be so taxing.
Weather effects also work the exact same way.
A PS2 can make the Beaucedine glacier snow storm so thick you can't see a malm forward in it but on a PC you can't see the effect correct even if you're running the game on full screen mode...
Architectural differences call on certain circumstances, you see ...
PC version could indeed use a overhaul.