I remember reading that one of the most g loaded tests was recognition time. I think the experiment involved flashing letters and timing how fast it took to press the letter on a keyboard. The key correlate was "time until finger left the home keys" which the authors interpreted as the moment you realized what the letter was.

I also heard a case that sensory memory lasts for a short a relatively constant time among humans and that difference in cognitive ability were strongly related to how speed on pushing information into sensory memory. The greater the speed the larger a concept could be pushed in before key elements started to leak out.