Well, of course, looking at the waterfall makes PlaneShift slower.
Yes, of course, but it is not the main cause (at least for me). The effect is the same for most portals. "Inner" and "outer" parts of bronzedoors map are in different sectors connected through big portal.
A little demonstration. It is in DR, but I get similar results also for Bronze Doors region. Look at FPS counter in left lower corner (displayed by bugplug)
1) libCg.so and libCgGL.so from last unofficial binary distribution for Linux, I can't even figure out, what version of Cg it is (files size does not match any of 1.2.1, 1.4.0, 1.4.1, 1.5beta1, 1.5beta2); result also applies (more or less) for version 1.4.0

2) libCg.so and libCgGL.so from Cg toolkit 1.4.1

43.31/6.49=6.67x more fps in case 2

. Nvidia GeForce FX 5600