Why is there a delay between two consecutive "ls" commands?

Issue

• We're encountering recently anomalies as there is a delay between we press "enter" and execution of the command in the command line.

Second time, delay is 22seconds

[root@server1 sa]# time ls
sa01  sa02  sa30  sa31  sar01  sar30  sar31

real    0m0.001s
user    0m0.001s
sys     0m0.000s

[root@server1 sa]#
[root@server1 sa]#
[root@server1 sa]# time ls
sa01  sa02  sa30  sa31  sar01  sar30  sar31

real    0m22.319s
user    0m0.000s
sys     0m0.001s

[root@server1 sa]#
[root@server1 sa]#
[root@server1 sa]# time ls
sa01  sa02  sa30  sa31  sar01  sar30  sar31

real    0m1.857s
user    0m0.001s
sys     0m0.001s
[root@server1 sa]#
[root@server1 sa]#

• The load average is a little bit unstable, right now it is:

[root@server1 ~]# uptime
 16:41:56 up 3 days, 58 min,  3 users,  load average: 6.30, 4.92, 4.32

• The IO wait according to say is at least 39%

• Machine was rebooted last friday around 15 (31/aug/2013)

• It's a virtual vmware machine.