Z buffers

The Z buffer itself improves performance on a per-fragment basis. If the GPU determines that a fragment is obscured (by comparing its Z to the Z buffer), then it will skip any further processing.

In addition, modern video cards have early Z rejection. If the GPU determines that an entire triangle is obscured, it will be skip the entire triangle.

if you would sort everything from most far objects and use BSP, PSV etc. you could achieve higher speed than with z-buffer just turned on. to increase performace with z-buffer it is best to sort objects from nearest to most far (if in reverse direction then it will need to render nearly everything and it can just happen due to unsorted objects)