In large part it's text. The graphics card's 3D engine basically can't render text characters, ditto bitmap graphics. All it can render is textured triangles. So to draw a text character the game has to create 2 triangles (which fit together to make up the rectangle of the character box) with a texture for each one that's the correct half of the character's image. Ditto for every UI element in every window. That adds up to a lot of triangle really fast. My guess is the UI takes more GPU resources to render than the whole visible game world. On a lower-end card that's not designed to render huge numbers of elements at once, it gets really obvious. You can turn down texture resolution, triangle resolution and such for the game world, but you can't do much about the number of elements needed to render the UI beyond removing visible windows.
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