Paolo posted
http://post-office.corp.redhat.com/archives/rhkernel-list/2011-July/msg00610.html

This request was evaluated by Red Hat Product Management for inclusion in a Red
Hat Enterprise Linux maintenance release.  Product Management has requested
further review of this request by Red Hat Engineering, for potential
inclusion in a Red Hat Enterprise Linux Update release for currently deployed
products.  This request is not yet committed for inclusion in an Update
release.

Patch(es) available in kernel-2.6.18-282.el5
You can download this test kernel (or newer) from http://people.redhat.com/jwilson/el5
Detailed testing feedback is always welcomed.

Verify this problem with 2.6.18-301.el5.

To test the kernel build time:
1. Install a host with kernel-xen 2.6.18-301.el5, and boot with the xen kernel
2. build kernel with the src rpm for three times and record the time:
# time rpmbuild -bb --target=x86_64 kernel.spec
3. downgrade the kernel to RHEL5.7 released kernel (2.6.18-274.el5)
4. repeat step 2

It uses less time after the fix, below is the result:

Step 2 (after):
real        56m7.636s          user        79m48.695s          sys        15m58.556s
real        53m39.934s        user        79m53.972s          sys        16m9.361s
real        52m45.541s        user        79m53.576s          sys        16m31.394s

Step 4 (before):
real        60m27.100s         user        84m13.988s          sys        13m50.640s
real        61m28.617s         user        84m22.500s          sys        14m1.193s
real        60m44.717s         user        84m21.696s          sys        13m58.276s

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHSA-2012-0150.html