ext4 : big directory entries produce slow ls output

Issue

• ls command takes many time on first run

# time ls /mnt &> /dev/null

real    1m1.896s
user 0m0.002s
sys 0m0.142s

• Any subsequent run is much faster due to dentries being cached

# time ls /mnt &> /dev/null

real    0m0.048s
user 0m0.001s
sys 0m0.046s

Not much entries are listed by ls