Why does `fgrep` take too much time for searching a string In Red Hat Enterprise Linux 6.2?

Issue

• Why does fgrep take too much time for searching a string In Red Hat Enterprise Linux 6.2?
• This is not happening with Red Hat Enterprise Linux 5.7 on the same file.
• On the Red Hat Enterprise Linux 5.7:

# cat /etc/redhat-release 
Red Hat Enterprise Linux Server release 5.7 (Tikanga)
# time fgrep -i <some_pattern> /path/to/file.dat
<output truncated>................

real    0m0.192s
user    0m0.159s
sys     0m0.032s

• On the Red Hat Enterprise Linux 6.2:

# cat /etc/redhat-release 
Red Hat Enterprise Linux Server release 6.2 (Santiago)
# time fgrep -i <some_pattern> /path/to/file.dat
<output truncated>................

real    0m6.361s
user    0m6.346s
sys     0m0.014s