I seem to recall seeing an article recently comparing several revisions of the Linux kernel but after a few minutes searching I didn't find it. The reason I'm looking is that today I upgraded a test machine from an old Debian Lenny release running Debian kernel 2.6.26-1-amd64 to Debian Squeeze running 2.6.32-5-amd64 and my test results are now up to 35% slower than they were before the upgrade.
For reference, this is an LDAP search rate test against OpenLDAP HEAD running on an HP Proliant DL 585 G5 with 4 Opteron 8354 CPUs (16 cores total). The LDAP database is 100% cached in memory, occupying about 25GB of the systems 64GB RAM. There are 4 client machines generating the load across a single gigabit ethernet. The search throughput maxes out around the 66,000/sec mark because the network interface is near saturation at that point.
At the lower load points, before the network interface gets overloaded, you can see that Lenny is significantly faster/more responsive than Squeeze. The OpenLDAP code is identical, built against Berkeley DB 4.7.25 and those binaries are also identical, so the only difference is in libc and the kernel.