Parallel PCI at the standard 33mhz bus speed is good for 133MB/sec in burst mode. (Some older Macs didn't run the PCI bus that fast.) 2048x1536 at 32 bit color translates to a whopping 12MB frame buffer so... sure, if you were running some sort of application that needed to change *every pixel on the screen at once* (Basically something that displayed a video or a game by rendering a gigantic bitmap in main memory and brute-force shoving it into the framebuffer without using any acceleration or rendering features on the card itself) you'd be limited to around 10 FPS. Of course, that's a pretty ridiculous worst case scenario.
(Any application that needed to push that much bandwidth would probably find itself limited in a whole slew of other respects trying to run on a PCI-only PowerPC Macintosh. I'd be sort of surprised if the main RAM subsystems in the Beige ones are good for 133Mb/sec.)