Bus bandwidth would only come into it if the video generator was pulling its framebuffer from system memory over the expansion bus, which I've never, ever seen. A Nubus video card would almost certainly have its own memory, and onboard video wouldn't pull it over the Nubus. And MacOS certainly doesn't rewrite the whole framebuffer every refresh - most games don't even do that.

Also, trying to generate video with a CPU (which, if I read your post correctly, is what you're doing) is a much heavier affair than using a proper video generator IC. You can pretty much do video generation with a DMA controller and a shift register.