Why are system calls so slow on Redhat 5?

Why are system calls so slow on Redhat 5?

My department is considering moving to RHEL5 from RHEL4 but we've hit a
problem. We have some software that executes a bunch of system calls in
succession and we've noticed that it takes much, much longer on RHEL5. The
scripts are written in bash shell but we're also seeing the problem in C
shell, Perl, and Groovy. An example:
foreach run (`seq 1 1000`)
echo `expr 1 - 1` >& /dev/null
end
This script takes .99s to run on a RHEL4 machine but 13.35s to run on an
identical RHEL5 machine.