But it still baffles me that you can have thousands of the same quad everywhere and it still be faster than one or two big quads.
I guess when they're far away you're only rendering a few tens/hundreds of pixels for each one, whereas for full-screen quads you're doing, eg. 1024 x 768 = 786432.
I can't explain how come a card can fill a full-screen's worth of polys (eg. the 3 walls you mentioned) yet slow down for one poly filling the same total area... must be some sort of exponential slowdown the larger a polygon is...? Then I guess you've got mip-mapping, etc, to take into account (ie. lower-res textures further away)...