Only one test may not be relevant, since the standard deviation of recording times is usually quite high. Also, the card might be warmed up on one firmware, and not on the other.
I know it takes time, but a more relevant test would be to list a larger number of attempts (say around 10), then change the firmware version, do the same test 10 times, then go back to the first firmware version, do another 10 recordings, then go back to the second and do another 10 recordings.