Normally I'd let this slide but you're testing different kernels here.
Cent0S is using 2.6.32: You're comparing it against Ubuntu 12.04 when you should be comparing it against Ubuntu 10.04.
At best, you should be comparing against Slackware 13.1 (2.6.34) and so on....
Linux 3.5 has made so many changes to the Ext4 file-system I wouldn't even know where to start talking about performance in relation to your test criteria.
Another thing about Slackware is that if you choose the default -- install everything -- then most likely you got the HUGE Kernel compiled i486.
You'd basically have to check /boot/vmlinuz to see where it's symlink'd to.
To recap: Cent0S 6.2 -> Ubuntu 10.04 [ 2.6.32 kernel ]
Cent0S 6.2 -> Slackware 13.1 [ never used the .32 stable ]
Ya Slackware kernel comes without Automatic process group scheduling "the miracle patch", and this has a major inpact on system,Slackware kernel timer freq is on 1000hz,I use Slackware about 10 years, i use and Arch linux too but on x86_64 they are no difference in speed .Speed on Slackware and Arch Linux is the same,Slackware programs run faster than Arch,Slackware is fast https://www.youtube.com/watch?v=nO5JR85_gWQ