Just timed it on level 1.
Exactly 10 seconds from the first appearance of the letter until the next letter appears.
Letter appeared, I started stopwatch. Letter disappeared. Letter appeared, I stopped stopwatch. 10.01 seconds.
*wait for letter to disappear again*
Letter appears, I start stopwatch, I run and grab letter then wait. Letter appears, I stop stopwatch. 9.98 seconds.
The difference of 0.03 seconds is most likely due to response time differing slightly, and if not then it is an extremely small difference to have any use.