why does first mkdir takes a longer time on ext3?

Issue

• First mkdir took more than 6 minutes on 16TB ext3 filesystem.

/disk6/RGC_special_data/shunsuke
07/30 04:09:14

real    6m29.810s
user    0m0.003s
sys     0m2.038s

• Second mkdir took less than 1s.

/disk6/RGC_special_data/shunsuke
07/30 04:16:34

real    0m0.521s
user    0m0.000s
sys     0m0.521s

• I reproduced the similar phenomenon on 512GB ext3 filesystem.
• After mounting the filesystem, first mkdir took around 20 seconds.