Not due to slow RAM, but due to slow CPUs. The CPU/memory latency gap wasn't such an issue yet at the start of the 90's. A 386 PC was pretty much the first cons

Not due to slow RAM, but due to slow CPUs. The CPU/memory latency gap wasn't such an issue yet at the start of the 90's. A 386 PC was pretty much the first consumer hardware where one could start thinking about doing any per-pixel work at all with the CPU in 320x200 at 1 byte per pixel. Also depth buffers only ever were viable with dedicated rasterization hardware, and on early consumer "GPUs" they were a luxury because even when depth testing was supported, they reduced rendering performance by half - which basically meant "utterly unplayable". It was almost always better to determine surface visibility through other techniques.