With Red Hat Enterprise Linux 5.x using the Linux 2.6.18 kernel, EXT3, and GCC 4.1 by default, there’s quite a number of areas in Red Hat Enterprise Linux 6.0 where the performance has improved measurably, assuming you had not manually built newer key packages for RHEL5.
Using GCC 4.4, EXT4, and the Linux 2.6.32 kernel results in noticeable improvements in many areas except for a few where the disk performance falls due to the EXT4 file-system, but that should be because of additional safeguards to protect the drive’s data.
Reference: Phoronix RHEL 6.0 Benchmarks