The problem isn't solely in rendering fast enough--it's end-to-end latency. Keep in mind that if your eye is being tracked by a 120Hz camera, that's 1/120 =~ 8.3ms latency just from inter-frame time alone. Add to that all sorts of communication bus overhead, image processing the eye tracking imagery to figure out eye direction, potential buffering in the display adapter, 5ms is not really possible, yet.

That said, I don't think there's theoretical limitations that prevent extremely low latency systems from happening, it's just not something we've kept in mind when designing modern consumer hardware.