Like when clicking on a file in a directory you just entered and looking for the file, the observer can literally locate and point to the file for the mouse user 5-10x faster than the mouse operator.
The observer seems to interpret the information that results from the directory listing faster than the person who just did the double-click to enter the directory because they don't have the muscle coordination context switch and can immediately move to interpreting the results.
It's probably because mouse manipulation uses brain infrastructure that is more recently evolved, but observe-react is a lot earlier in the brain processing pipeline evolutionarily, and a lot more refined/involved.