I don't - just filmed keyboard and screen together; key down is easy to see. I have a quality keyboard (Fujitsu KBPC PX eco) connected by PS/2, and as stated above I'd expect little extra latency from USB. As measured, anyway, there is no space for significant keyboard lag in the result. The reason why I measured latency was that I seemed to notice a change after changing GPU driver kernel options. End to end was easiest to measure. The result was close to the theoretical minimum so I stopped there.