Kworker is a backgrounded kernel process, its a placeholder for other kernel related processes like i/o and timer functions.
there is an open bug report https://bugs.launchpad.net/ubuntu/+source/linux/+bug/887793
(workaround on post #116) https://bugs.launchpad.net/ubuntu/+source/linux/+bug/887793/comments/116
where Kworker sucks ~100% cpu, 2% seems normal?
solutions seem to range from upgrading kernel to disabling a certain interrupt, gpe13
ps: If you really want a fine tuned machine, ubuntu based distro's are not the way to go. You may want to check out 'Arch' or 'Gentoo'