The ZX Spectrum had a slight advantage for early 3d games because of its simple design, not despite it. Not much in the way of custom graphics hardware, screen modes, and sprites. Just directly set the values in the memory of the display buffer. I think this was also an advantage of the PC when it was up against early consoles and 16-bit computers with their custom graphics chips optimized for 2d gaming. Then this was combined with the 32 bit Pentium processors in PCs at the time.