[crest]

Z-buffers don't just require about an other frame buffer worth of memory, but also lots of read and write bandwidth per pixel. This extra memory bandwidth requirement made them expensive to implement well. High end implementations used dedicated RAM channels for them, but on lower end hardware they "stole" a lot of memory bandwidth from a shared memory interface e.g. some N64 games optimised drawing a software managed background/foreground with the Z-buffer disabled to avoid the cost of reading and updating the depth information.

[rasz]

Power of Z-buffer lies in letting you skip all of the expensive per pixel computations. Secondary perk is avoidance of overdraw.

Early 3D, especially gaming related, implementations didnt have any expensive per pixel calculations. Cost of looking up and updating depth in Z-buffer was higher than just rendering and storing pixels. Clever programming to avoid overdraw (Doom sectors, Duke3D portals, Quake sorted polygon spans) was still cheaper than Z-Buffer read-compare-update.

Even first real 3D accelerators (3Dfx) treated Z-buffer as an afterthought used at the back of fixed pipeline - every pixel of every triangle was being brute force rendered, textured, lighted and blended just to be discarded by Z-buffer at the very end. Its very likely N64 acted in same manner and enabling Z-buffer didnt cut the cost of texturing and shading.

Z-buffer started to shine with introduction of Pixel Shaders where per pixel computations became expensive enough (~2000 GF/Radeon).

[kevingadd]

Some modern algorithmic smarts (like hierarchical Z) helped a lot to reduce the memory bandwidth demands, to be fair.