How long does the export take? I'm assuming it's taking less than 5 minutes for a dump of 15 meg compressed. If this is correct, my opinion is that it will not make any difference whether you do it at 2am or 11:pm .
I don't really know, but I guess that your Operating System resource is shared by many other processes at 9:00 , so it takes too long.
If you find why please let me know.
I will search the answer by myself.