Why exactly should latency be such an issue for graphics cards? If the graphics card only needs to refresh the screen every 15ms what difference would a few na

Why exactly should latency be such an issue for graphics cards? If the graphics card only needs to refresh the screen every 15ms what difference would a few nano seconds make for the signal to travel down a wire? I'm guessing that reading to PCI express if already >100ns for large payloads.